Tell insurance companies it is an RV. In your case a Thomas RV (or whoever the company is). Be vague. Do not use the words bus or conversion. Worst case scenario is you'll have to find a broker to insure it at a higher price.
You could also try registering it as a commercial vehicle and get it insured that way.
It's a PITA. If I had to do it all over again I'd get it registered as commercial or a van. Mine is registered as a motorhome and they don't do commercial insurance on those. RV insurance is cheaper but it is just tough to find someone to insure it.