There are two scenarios for insurance:
1. You bought a bus that's titled as an RV/motorhome, AND the conversion is completely done.
2. Everything else.
Let's start with the first scenario. Your best bet is just to start calling insurance companies, and see who will give you coverage at the best price for a converted bus. This could take a while, and varies from state to state, and person to person. So just keep trying, and hit up some of the forums and Facebook groups to find out what's worked for people in your area. BE UPFRONT with the insurance company about what it is you have. The worse scenario would be to have coverage, get in an accident, and then the insurance company refuse coverage because you misrepresented what the vehicle was. Often the insurance companies will require an inspection or photos of the completed conversion.
The second scenario is easier—although some people make it harder than it needs to be. If you just need to get the bus home, just get commercial insurance on a private vehicle for your bus. Progressive Commercial has no problem insuring your bus while it's still titled as a bus even. It'll be more expensive than RV insurance, but even $600-800/yr isn't that expensive if you're just using it for a month or so while you move it to where you need to park it, then cancel your policy.
Get Coachnet or some other similar roadside assistance, especially if you're unsure of the reliability of your bus. Towing a bus is VERY expensive. As in a 100 mile tow would be around $1,500 (don't ask how I know that)