Already good answers here. Two (2) things again. This may be a situation where your personal agent may be able to help you bind your Bus Conversion. Also, like others have already said,...Progressive Insurance specializes in this kind of stuff.. $Costly$, but doable.
They insured my '74 Crown Super Coach 40-foot 10-wheeler ex-schoolie. Yeah, again do not say the words "conversion" or "homedone/made". In my case my Crown had less than 1000 miles on it since a CA CHP school bus inspection. Good luck.