I went and got my NY R Endorsement Permit today. Process was very easy.
1. Filled out MV-44
and brought it in. You are applying for a "permit" (even if you already have a license), because you want to "change type". In the section labeled "other change" say "Add R Endorsement."
2. Bring mv44 to the DMV where they will have no idea what to do
3. Patiently wait while they talk to people into they figure out how to issue you the permit. they'll eventually have you retake the eye test and send you to pay. It costs $10.
4. The permit entitles you to: a. drive an overweight (>26000 lbs GVWR) RV with a licensed driver for that size vehicle, either another class D with and R endorsement, or a class B or A CDL, and b. It entitles you to two road tests.
5. I haven't done this, but the last step is to head to your road test, you can do this online once you have your permit. No drivers Ed necessary. Just schedule at the nearest RV test site, 74 miles away for me... In Ithaca. Good thing I love visiting Ithaca... You must
bring your licensed driver. Their license will be checked.
The test is identical to your class D test. You do not need to do anything special for Air brakes (confirmed at the DMV today), but your test admin might ask questions anyway. It's a good idea to learn to do the test anyway I plan on it.