Someone here had mentioned being able to look at the frame for excessive rust or frame damage, hence removing the belly pan. That seems a bit much if your looking around as not to many sellers will let you remove their belly pan to peek underneath. However it does make sense because those are the biggies. Floors and frames once you know they are bad you can't really ignore them.
There is a good thread here somewhere that talks about project paralysis. Know yourself and how you dive into a project. Is time or money an obstacle. Are you more of a perfectionist or easy going. Its very similar to an older house. One fix is usually tied go another and another and another.
Our guideline was to keep it camp able and not sweat the small stuff. Right now we have a 35 yr old house and two 50 yr old Airstreams, when it rains outside it also rains inside....all three.
There are quite a few gutted shells on the classified where floors or frames may have been done already.