Congratulations on your purchase and welcome to the forum. You do not actually need a RV dealer and would probably be better of at a truck mechanic. What you have is P-30 GM Chassis with a 454 GM large block engine, coupled to a three speed transmission. If I was you I would take it to a GMC truck dealership. Now you mentioned that it has sat for five years and that is about the life of a tire. Even though they might look good the potential of dry rot is high and you might want to replace them before driving across country.
Good luck to you and keep up updated on your progress.
1994 30' Excella 1000, Chummy III- Ford Excursion- 7.3 Turbo-diesel