I have the definitive answer on my insurance dilemma in my state and my county from my insurance company (farm bureau).
A commercial policy will cover a bus. I told the truth - I usually say more than I should
- to my agent and his underwriter does not have a problem with insuring it as a "bus" under a commercial policy. It costs $330 for 6 months - liability only. My state is CHEAP for insurance and I have no issues like tickets that bump me into a higher premium bracket. Expect to pay double if the company can find a reason to put you in a less "preferred" tier. Once it is converted it will segue over to my personal auto policy.
Insurance in general is tricky business - they evaluate "risk" by credit score, driving record, "exposure" - ie ferrari verses honda and how many lines of business you have with them. It is an industry that can be relationship driven - they like you better if you insure a home and your cars with them!
ps - I did hold a licence to sell property and casualty (home and auto type) insurance in Florida years ago so have some insights into how it works.