Before you go too far with this, call around to the various RV places and see if anyone is going to give you a "certified" inspection. I'd be a little more concerned with what their inspection covers and what they will do to remedy a problem they didn't find and how long you have to find any problems they didn't find.
I expect you'll find someone to inspect it, but they probably won't stand behind their inspection - meaning they won't pay for anything they didn't find.
And remember an RV inspection is just like a house inspection. Some things might need to be fixed for real safety issues, but the rest of them are things leading up to the overall condition that the current owner will tell you are just wear and tear, fix them yourself if you want them fixed.
As for expense, it's your responsibility to transport the trailer for the inspection or pay the inspector to travel to the trailer, not the sellers.
And as for buying from a dealer, the only things you can expect them to fix or cover are things you find before you leave their lot - just the same as leaving a private sellers drive way. You'll have to buy some sort of aftermarket repair policy which they will be happy to sell you if you want further coverage.
I just bought a used trailer from a private seller last month and a used DP from a private seller four years ago. I generally prefer to deal with private sellers, but buy wherever you find the best deal.
Bill
Nodwell RN110 out moose hunting. 4-53 Detroit, Clark 5 spd, 40" wide tracks, 10:00x20 tires, 16,000# capacity, 22,000# weight. You know the mud is getting deep when it's coming in the doors.