This and insurance are the two biggest question all new bus owners face. Unfortunately, every state is different so you won't get a solid answer from us most likely.
Many states have several options. In Minnesota I could have registered as a private bus, Y-plate weight based registration, or as an RV. For me, registering as an RV was the best option. While it was the same price as the Y-plates (they use the same tax base), I get the insurance benefits and no questions at campgrounds. Some people have better luck as a private bus.
What states require varies widely. I simply had to sign an affidavit with several categories. It was not unlike getting a degree in college in that there were categories and you had to have a certain number of points in each category. For me this meant that I had beds, potable water, a toilet, 12volt AND 120 volt wiring, and a permanent propane fired cooking appliance. The rules looked strict, but it was all stuff I had (or was going to have...I technically was a little short when I registered) and they didn't require an inspection.
You best bet will be to contact the DMV, DOT, or State Patrol office in your area. In fact, contact several. It's good to know what the DMV requires from a legal standpoint, but it's also good to know what the State Patrol is out looking for as well.
As for insurance (that is generally the next question after registration), the general consensus on the board seems to be GMAC or State Farm. I personally have State Farm for all my vehicles as I've known my agent for a long time and she takes good care of me. It was no issue for her to register it. The only questions she asked we more out of curiousity. I pay about $200 per year, unlimited mileage. As the bus sits right now, the insurance is suspended. Should I want to drive it at any time I just need to call and leave a message....done deal. My insurance includes roadside assistance, a worthy $5 or $10 per year. The cheapest towing place I've seen charged $250 for hook up and $5 per mile for a vehicle this size and that assumed I would be caging the brakes and removing the axle shafts myself. While I can handle the brakes, I sure don't have a slide hammer big enough for the axles.
The best I can tell you is start calling around. Make sure you have pertinent information handy such as odometer readings, chassis make, body make, etc. Pictures are also helpful although at this point I doubt you have much to show.
Best of luck!