Originally Posted by DocBrown
When its folded up check the door to see if there is a V shaped gap between the door edge and the trailer wall. That would not be a deal breaker for me but it's an indication that the frame is flexing. The question is how much?
Careful with this one or you may never buy a pup at all. There is a flare engineered into the door frame on most pups, and certainly on Jayco. This is to prevent door binding and make installing the main door easier. Now if the gap has opened up more than reasonable (say to the point where you have daylight past the half-door), you might have an issue. expect the door opening to be about a half-inch wider (guessing, never thought to measure it) at the top than at the bottom.
I would also pay close attention to the axle camber. Crouch down behind the camper and the tires should toe in at the bottom when the trailer is unloaded (axle has a slight bend upwards in the middle). When fully loaded this should compress down to almost flat. If the wheels toe out, or the axle crowns down, then the axle has been bent from stress or overloading. This is also evidenced by uneven tire wear.
And as everyone else has said, beware of water infiltration, the enemy of all RV's. Check for signs of damage and proceed accordingly. The good thing about a pup is that almost everything is visible to the naked eye, but you have to open cabinet doors and check inside bins under the seating. Lift up the curtains and skirting as well.