In Michigan in CDL exceptions it says:
Individuals: Operating motor homes or other vehicles used exclusively to transport personal possessions or family members, for non-business purposes.
I think a bus clearly fits that description, regardless of weight.
You will need a CDL to drive it after it's a commercial food truck. You won't as an RV need a CDL, but I don't think you'll be able to sell food out of your "RV".