Call your state's licensing entity (or State Patrol) and ask them what license is necessary to drive a motorhome
. Once they tell you ask them if there are special endorsements for air brakes
on a motorhome. I've found that if you don't confuse the issue by mentioning a school bus that it's much easier for them to answer the question; you're not hiding a darn thing as when converted your rig will be a motorhome. It's just that if you mention "school bus" they automatically jump to conclusions about its use and give you answers on the commercial
So far (and I'm not a nationwide expert) I've not heard of a motorhome requiring a CDL; that's a Commercial
Driver's License and you're not driving a commercial
(for hire) rig. I have heard of some states that require an "air brake" endorsement on your regular (private) driver's license if your rig does indeed have air brakes. Staggerlee917 may have gotten 'right on' info for Colorado but it sounds more likely that they gave him info required to drive a "School Bus" with air brakes.
To the best of my knowledge as long as your bus is still a "School Bus" (and licensed as such), regardless of private ownership or intent to convert, you are required to have a CDL-B license with an air brake endorsement (if it has air brakes).
My bus isn't a motorhome in Washington yet because it hasn't been converted (and inspected); it is licensed as a private vehicle (I carry the same tags as an automobile) and I do not have to have a CDL. In general, unless you drive a vehicle over 26,000# (that is not a Recreational Vehicle (or farm or fire-fighting equipment in some locales)) or carry more than 15 passengers you should not be required to have a Commercial Driver's License...but...YMMV.
It's a confusing issue; the basic requirements for CDL's are set at the Federal level and every State must comply and license the same. But States can (and do) add "endorsement" or "waiver" regulations; those are the ones you'll have to research for your particular State.