what do I have to do in order to be able to drive that bus in NY and elsewhere
I assume you mean with NY registration and NY license. If you leave your home state, other states honor the privileges granted by your home state license.
The answer depends if it is 26,000 pounds gross weight* or less, or over 26,001 pounds gross. DMV info on license classes is here:
If you do not charge passengers fees for rides (cost sharing excepted), it is under 26,001 lbs gross weight, less than 45 feet long, and does not have air brakes, just register and drive it. You then have fifteen days to paint it other than "National School Bus Chrome" (yellow).
Despite the 18,000 lb. limit that might be printed on an older licenses, about two years ago the legislature added "non-CDL C" class privileges to all Class D (passenger car) and E (limo/taxi) licenses.
If it has air brakes, you may be able to add the endorsement to a standard driver's license, but I have never heard of it being done. You'd have to ask at the DMV or County Clerk's office where you get your license. The endorsement was just an added written test when I got my CDL learner' permit, but I don't know if it applied only to the permit or went right onto my license privileges.
If it's 26,001 lbs or more and not a "House Coach," you will probably need a Class B CDL and pass a DOT physical. If it's 26,001 lbs or more, or over 45 feet long and IS a "House Coach," you will need an 'R' endorsement on your regular license. By Federal law, states cannot require an actual CDL for an RV.
I thought I had seen the list of requirements for "House Coach" registration in NYS Vehicle and Traffic Law, but I can't find it today. It was a typical requirement that the vehicle must have a certain number from a list of options, (sleeping, cooking, water, toilet, power separate from engine, heat separate from engine, etc.) but as I recall, the options had to be built-in. A cot, Coleman stove, ice chest, jug of water, port-a-potty and a portable heater didn't cut it here.
*NYS VTL 502-A says the license required goes by the heaviest of either manufacturer's design weight (data plate), registered weight, or actual loaded weight.
EDIT: OOPS!! I accidentally typed an "R" endorsement was required over 5 feet instead of 45 feet.