OK I'll take a stab at this one, having been there and done that. Get yourself an ice pick. Open the lower cabinets. Pull back the carpet a little and poke the icepick into the subflooring. Solid means probably no water damage, hence probably no frame rot.
Place that are most vernerable, bathrooms, does the bath room floor flex at all? Poke under the cabinet with your icepick, does it look like its been leaking around the toilet? If so assume more rot.
Roof vents also leak. Check the floor directly under the AC unit and Vents if its carpet poke it with your ice pick, OK maybe owner won't like that but, your are the one spending the money.
Also in front of the door, water leaking in under the door rots of course the floor and then migrates down onto the frame.
There really is no other way to check the frame unless you remove the bellypan.
And done be afraid to ask questions, some of mine have been pretty dumb, I didn't even know what a blackwater tank was when we first started our Airstream journey