The bus has been converted and should be registered as a non-commercial recreational vehicle, so it would be the same license for that... probably a Class A, depending on gross vehicle weight rating.
It's a slight step up in training from your normal vehicle, and would probably require a health certificate, etc... but not as expensive as a CDL (commercial driver license), which would usually entail a few weeks of training and ~$2,500 in costs.
Disclaimer: I own an RV.
We're building military 6x6's into motorhomes, and as soon as they meet the requirements for a living space, there's no special regulation.
Little hard to swallow the fuel cost if you're planning on back-and-forth trips, but my route is a long, drawn out squiggle across north and south America, so we'll only be doing a few hundred miles a month. And, hey, no rent to pay!
My employer provided training and I had to pass written and operating/driving tests in addition to medical exam/requirements. This was in Washington state. Similar in the several other states I have lived.
There are extra restrictions for commercial driving.
A 15 passenger van requires no extra license, but that's about the largest you can go.