In a Class C I'd look for rust in the fenders, door panels, and compartment supports and braces. More critically, I'd check the condition of the roof and any signs of leaks, especially in the front section over the cab. Leaks lead to rot and mold, both issues that can make a rust underneath look minor. A common issue is a leak around the windows over the cab, front and sides that often get ignored when checking for caulking failure because they're a little harder to inspect. Carefully inspect the interior and ceiling of the over cab area and be very suspicious of any stains, odors, or redone areas.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI