so, back in 2010 i bought a 1995 Ford Carpenter 66 passenger school bus for $1,800 on ebay. it was a wonderful machine that didn't give me any problems over the two years i had it.
getting insurance for it was a bitch, because i didn't know what i was doing. i'll save you some of that hassle.
first off, like creature said, register the vehicle as a 'Recreational Vehicle' (RV). You MUST do this, otherwise it's going to be incredibly difficult to insure the vehicle. if you have to take it to the DMV just explain to them that it's a bus conversion and you're turning it into a motor home / RV. they won't care as long as you pay the fees to register the vehicle.
now, also like creature said, GMAC is one of the few insurance companies that insure bus conversions. the fucked up part is that depending on the customer service person you get, they will either confirm or deny this. if they tell you they don't just hang up and call again until you get an agent that says they will insure a bus conversion.
i actually didn't have much luck with GMAC at first, but i got lucky and found a company called poliseek, which is a 'reseller' for GMAC. see in the grand scam of things in the insurance world, almost all insurance companies are owned by a handful of corporations. they give some of these insurance companies different names to give the 'illusion' of choice/competition but if you've never heard of the insurance company they're probably just a reseller for one of the big companies.
moving on. the agent at poliseek agreed to insure my vehicle through GMAC as long as all the seats were removed. now, at the time i was still in the process of removing the seats, but i just lied and said they were gone. they just want to know that you're not going to go around running an uninsured party bus, or transport people around commercially, since that requires a whole other load of insurance policies.
they might also ask you some questions about whether you're going to install a toilet/shower/stove, since those are things insurance companies worry about (like you blowing your vehicle up). i don't remember the specific questions about this, or my answers, but i just basically said i wasn't planning on installing that stuff and just using it for hunting excursions or something like that.
when it was all said and done, i ended up paying about $250 every six months for insurance.
DO NOT, i repeat, DO NOT go through progressive. they're fucking morons. before i went through the above, i got my insurance online through them, which was quick and painless, but then in my second month they cancelled my insurance, telling me they don't insure 'bus conversions' (even though i clearly stated it was a conversion, i was totally upfront about it). worse is that they fucked up the payment refund, so they sent me a check for the money i'd already paid (a year's worth of insurance in advance) that took TWO MONTHS to arrive. so yeah, fuck that company.
i've actually written on this subject before, i'd advise checking out the vandwelling section and searching for any posts ive made there.
also, i learned alot about school bus conversion insurance from skoolie.net which is a great forum for school bus conversion enthusiasts.