Jan,
Most people just buy one they like. Sometimes they get unpleasant surprises. Do you want to become expert at inspecting them? Or would you rather take your chances?
IMO the biggest issue with used RVs is water damage, usually hidden. Maybe the owner doesn't know about it. Inspecting for it is sort of a holistic thing. Water can get into the body anywhere there is a seam, or an item installed in a hole in the body or roof. So you look at all the seams, caulking etc. Including on the roof. If you spot a suspect area, then closely inspect the body inside and out near that area.
Go over the body and interior without skipping any spots. Meaning, open all cabinets, move mattresses out of the way, etc. Keep looking for hidden water damage.
As to your specific question. Trail Lites were built with pretty good specs and components for a low price. From what I've found, the weak point is their caulking and sealing. ON MINE, they didn't use a good caulk where the roof meets the gutters on the sides. I pulled it out and re-did it.
Here is my post. You can browse a couple of pages before and after that one too.
As far as crawling underneath:
And also on mine they didn't use good practices on the inner walls below the floor. What they did was use black poly sheeting to cover the walls where they hang down. Three problems:
1) They squeezed an aluminum channel over the bottom of the wall, and the channel is supposed to go up over the sheeting and hold it. But in some cases the sheeting is not in the channel. It is loose and the plywood wall is exposed. The fix would be to buy the Flex-Mend tape, clean the area, and tape over the problems.
2) The screws in the bottom flange of the small service doors. The fix is to remove the screws, clean up area, put caulk under the screw, screw it back into place, and caulk over them a bit for good measure.
3) The sheeting is loose behind the service door openings. Now, they did roll some rubbery undercoating around the holes in the body panels. It probably bought some time but it is breaking down. So the fix is to seal the loose edges of the sheeting to something. Either to the rough opening in the body panel, and/or use Flex Mend tape to tape over the loose edge and seal it to the actual white door frame (back side). BTW I'm planning to do this fix from the outside, through the doors, not from underneath.
So, let's say you crawl under and find the things I mentioned. If so, then push on some of the walls under there. If they are still solid, you're fine. Just do the fixes. No, it's not fun working under there but it's just enough room. If the walls are not solid, then you have to decide if you want to deal with the lower wall panels falling apart.
This is only my experience with one MH, my Trail Lite. I have no idea if other brands should be crawled under to inspect. Wouldn't hurt though! I will say, after 8 to 10 years of age, these things will really start to show any structural issues.