First check for me is to get on hands & knees to check on spring hanger to frame area reinforcement. At the same time a count of spring leaves is worthwhile. For a 5er, next check is the pinbox to main part of frame geometry. Too many of them flex too much & don't have the reinforcement needed.
I want to know freshwater capacity. If it is over 80gal only them will I take a peek inside. Less than 80gal is an automatic scratch from the 'possible' list.
Just these two must haves cuts the size of the pack of possibles down in a big way real fast.