Walking is ok on the roof, most RV's tout a "walkable roof".
You can even feel soft spots when you walk up there. Make a note of where such places are, and when you go inside the 5er, push up on those areas to see if they feel soft or "crunchy"...or are water stained or recently painted. Big bells should be ringing in your head at that point!
The edges where the roofing membrane rolls over the roof and is fastened under is a place where the TPO/EPDM is under the most stress and can crack or get holes.
And around all the intrusion points such as the roof vents, pipes, antennas, etc are the other point of water intrusions. And the most insidious place that most people ignore is the clearance lights. And the window caulking too.
This roof vent caulking is cracked and may have a leak!
Also crawl under the RV and look at the frame...broken welds, rust, dents, bald areas on the tires are all warning signs too.
And check out all of the propane and water systems. Good luck!
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Inspect all of the caulking, make sure its in good repair. Check all of the plastic items on the roof, make sure they are not cracked. while up there its a good time to check the top of the awning connecting to the unit.