Plumbing can be tricky on a bus. Make sure to consider placement of all your components before you commit to a layout for your bus. The Wheel wells, exhaust, and driveshaft can all complicate your tank-placement plans.
You can keep your design super simple by going with a composting toilet (which eliminates the need for a black water tank, as well as reducing the amount of fresh water you'll need, and eliminates the need to run a water line to the toilet). If you can get by with a single sink, that'll simply things even more.
Adding in a normal RV toilet means adding a black water tank, which MUST be located directly under the toilet. You'll also need to add a water line to the toilet. The below diagram adds the toilet, as well as a bathroom sink.
Laying out your bus plumbing
If you're using a normal RV toilet, you'll need the black water tank directly under the toilet. make sure there's not an axle, exhaust pipe, or driveshaft where you'll need to place the tank
The top of your gray water tank has to be lower than all your other plumbing...so make sure you can slope all your drains down to it (sometimes the frame will get in the way if you're locating it across the bus from a sink or the shower).
Use a Hepvo trap instead of a p-trap. It can be used horizontally or vertically, which might save your bacon in tight install spaces. Another bonus is that it's not subject to sloshing out when driving, or evaporating when sitting unused for long periods. I'd recommend you use one for your sink (or sinks) and a separate one for your shower. Using just a single hepvo trap could result in water draining from your sink backing up into your shower in the event of a hepvo trap clog.
When you purchase from any of the links below, I'll earn a small percentage which helps cover the cost of running this site!
What kind of plumbing should I use?
The general consensus is that PEX is the way to go. Flexible, durable, and easy to run—and using SharkBite fittings makes it easy to assemble. Make sure it's all well attached, and doesn't have rub points with sharp edges where it passes through the bus body.
Where do I get tanks?
If you're lucky, you'll find some on Craigslist, or maybe scrounge some off of a junked RV. You can also use 55 or 30 gal plastic drums, although they might not make the best use of space, and take a bit more work to add fittings (but hey, they're cheap!). If you're not so lucky, you'll need to buy new ones...and you might choose to purchase a new one for your fresh water anyway.
Buying them new is a lot more pricey - a 100 gallon tank will run you between $150-300...and shipping could double that. One of the best places I've found is RV & Van Surplus, in Elkhart, IN (you can pickup there in person to avoid shipping if you're in the area).
How to mount the tanks
Make sure you do this right! You don't want a 100 gallon tank (that's almost 900 lbs!) coming lose. Whether they're inside the bus, or underneath, make sure you're accounting for the weight of the water when you secure the tanks. A bungie cord is not gonna cut it. Make sure you've got the tanks not only secured on the bottom, but also front to back and side to side. Those braces need to be able to take the full weight of the water slamming forward into the front of the tank in an emergency stop.
Inside mounting, especially for the fresh water tank, is a popular option for people who will be living on their bus in sub-freezing temperatures. Under your bed or couch makes a convenient out-of-the-way location. You can get away with the gray water being underneath, especially if you use at least some insulation. The warm water dropping in from the shower/sink will serve to keep it from freezing longer than you think. Mounting a gray water tank inside is especially tricky, since it has to be lower than your lowest drain...which is pretty hard if you have a shower.
In the US, it's standard to have the waste valve on the driver's side of the vehicle. If you don't follow this rule, you may have to back through some dump stations to empty your tanks.
You've got multiple options when it comes to water heaters. The first decision is whether you want propane or electric (or there are some combo water heaters out there that do both). Or maybe you just want a bare-bones direct solar water heating system?
This is the most common—you can find normal RV 6gal or 10gal ones pretty easily for $200-400. Or tankless options range from the popular EccoTemp models designed to be used out-doors (so might need to be vented), and available for as low as $100, all the way up to the $1,000+ PrecisionTemp, which is actually designed for RVs, and even comes in a floor-vented model, so you don't have to cut a hole in the side of your bus.
If I were going propane, I'd probably lean towards the EcoTemp L10
If you've got a really good solar system, or you're planning on being plugged in all the time, you might want to consider using a small 15-20 gallon residential water heater on a switch. If you're running on solar, turn it on, give it 20 min to heat up, take your shower, then turn it off. And it'll keep water warm long enough for hand-washing and stuff for several hours. If you're trying to avoid propane, this is your ticket...but fair warning, you'll need more than just a single 80w solar panel for this setup. The one success story I know of with this setup has around 1,000w of solar power and a large battery bank.
If you're the DIY type (which you obviously are, since you're reading this website) then maybe setting up a roof-mounted PVC direct solar heater. There are multiple designs (just google "diy pvc solar water roof" to get started)
Everyone's build is going to be a bit different, and I can't fit EVERYTHING into the diagrams, so I'm gonna just list a few options that you may or may-not want in your build.
Water pressure regulator on your shore water input. Just in case you hook up to some city water that has higher pressure than your plumbing can handle, this will keep your connections from blowing out, causing a huge mess inside the bus.
Bypass shut-off valves for the water heater are another optional setup that some people choose to have. This allows you to bypass the water heater if you ever need to work on it, so you don't have to shut off the entire system. A couple of T fittings and 3 manual valves is all you'll need to add this in...but it'll only be used in the event of a water heater failure, so a lot of people don't bother.
Whole-house filter—if you're going to add one of these in stick it in after the water pump and shore water inlet, but before the water heater. Make sure you can access them easily, that you install manual valves to isolate the filters while you change them, and that if water leaks out while you're changing them it's not in a spot that will matter too much.
Flush lines for gray & black tanks are another optional add-on. Install sprayer heads in the tanks to clean them out. Some people run hot water to these sprayer heads, to allow them to thaw the tanks in case of one of them freezes.
One-way diverter valve from gray water tank to black water tank. Since gray water tanks tend to fill up first, you can install a manual valve, with a one-way valve after it to move water from the gray water to the black water tank. Also handy for flushing out the black water tank.
Air admittance valves can be used instead of air vents behind the HEPVO traps. This keeps water from overflowing out of the vent in the case of a clog (or just make sure your vents are well-above all your drains).
Extra vent on fresh water tank. If you get "glugging" when filling your tank with the vent that's built into the fill line, you might want to add a second vent right off the tank.
Gauge those tanks!
Although tank gauges aren't technically necessary, it sure is nice to be able to check in on the levels of water in your tanks. Having your gray water tank overflow is not a pretty sight!
Traditional tank monitors use probes to sense the water level (which are somewhat susceptible to getting gummed up with sludge, impairing their ability to function correctly). Newer probe-less designs are also available to get around this problem, such as the Garnet Seelevel series.
Adding an outdoor shower
While you're running lines for plumbing, might as well add an outdoor shower as well. Not that much money, and could come in super handy for washing off muddy boots, muddy bikes, muddy dogs, or Muddy Waters... if he was still alive, and happened to visit your bus.
Accumulator tank? Do I need one?
Some people will recommend an accumulator tank. This is installed after the water pump, and reduces water pump cycling and smooths water faucet pulses. Other people argue that with a good-quality water pump, this isn't totally necessary. But it's not a major expense.
Drinking water tank
If you don't want to worry about keeping your fresh water tank perfectly sanitary (ie—you want to be able to fill it from streams, lakes, etc when boondocking) you might want to consider using standard 5 gallon bottles for your drinking and food prep water. These are easily found at grocery stores almost anywhere you go. One slick solution is to design brackets in your under-bus storage bays (if you have them) that will hold two standard 5 gallon bottles. Then use an electric pump designed for use in standard 3-5 gallon water bottles, pump the water up to a drinking spigot on the sink. These pumps auto-shutoff when the first bottle is depleted, and you switch the hose over to the second bottle, and replace the first one sometime before you finish off the second one.