Looks like you live in one of those "crazy" states when it comes to registering an RV! Some tips:
1) NEVER tell the DMV folk it's a BUS!
What you're trying to register is a 1966 MCI Class A diesel-pusher MOTOR HOME.
As soon as you say "bus," they immediately think Greyhound or skoolie, and thus, commercial vehicle - a whole different can of worms! (For reference, NH Title XXI, Sec 259:58-A Motor Home definition.)
2) Since your coach is over 25 years old, you are not required
to title the vehicle, except by owner's request. (http://www.nh.gov/safety/divisions/dmv/title/apply/exempt.htm
) Your choice. . .
3) The old title says 50K lbs? That's crazy. Run it over a set of scales to get the actual weight. (FYI, 32K is considered normal GVW for a two-axle coach. 12K front, 20K rear.)
3a) Ask if RVs are exempt from weight registration fees - those are normally associated with commercial vehicles, but each state's different.
4) Have no idea where you're going to come up with the original selling price for a 1966 MC-5A, especially since Greyhound generally bought them "in bulk" and they owned the factory, too. Maybe someone on the Yahoo MCI busnuts group might know. . . OTOH, perhaps they'll accept the Bill of Sale the PO prepped for you.
5) CYA by getting a "trip permit" from FL to go along with the Bill of Sale.
6) If you really want a confusing answer, send an email here: NH.DMVHelp@dos.nh.gov
and ask this simple question: "How do I register a 35-foot 1966 MCI Class A diesel pusher RV/Motor Home that I just bought in FL? It weighs [put scale weight here], and I have no way of knowing how much it cost new in 1966."
7) Since it's over 25 years old, try for antique plates!
Main point = #1 above!!!
FWIW & HTH. . .