Tips on Buying a Used Travel Trailer

Hey guys, I'm looking at buying a used travel trailer. I have no idea what to look for besides the glaring obvious things. Can some of the experienced camper owners give me some direction? Looking for a 20-28ft range bumper pull. Is the fiberglass models better than the aluminum skinned ones? I see there are aluminum framed vs. wood frame? Should I stay away from slideouts? No clue here.

Thanks!