Also look intently at the ceiling and the side walls of the roof for any bubbling of the vinyl cover, any signs of rot or water intrusion on the wood. Underneath whatever exterior covering is on the outside (typically aluminum or fiberglass of some sort) is usually plywood to make up the side walls of the roof. Check over all of that.
Look for water leaks around any plumbing under the sink and other water fixtures, including the pump and water heater.
Look for signs of mold in the corners, under the beds on the OSB/plywood bottoms, any place there is canvas/vinyl.
Walk on all areas of the floor and look for any place that gives. There will flexing and that is natural, but if something feels a bit squishy, check it out on the underside as well.
BTW, if you're not sensing it - water damage is often the biggest factor in longevity of PUPs.
Look for holes in the vinyl or canvas fabric.
Make sure all appliances work, demonstrated.
Overall condition and then some specifics to pay attention to. Tires are a commodity and need replacing every 4-5 years regardless of use. Brakes, if equipped need inspection. Bearings need greasing - when was the last time the seller did this?
Check the roof - is it sloping towards the middle, either front-to-back or side-to-side?
There's a host of other things to consider but you just have look at it closely and as best you can. Like I said, water is the most common failure reason of many aspects of a PUP.
