I have had 2; a 30' Shasta and a 30' Itasca. They seemed pretty much even, the Itasca was probably a bit better-made and stouter. Make sure the fundamentals work, like the AC, stove, furnace, etc. Take no chances with the heavy systems, it's really friggin' expensive to replace some of that stuff. Pay special attention to the water heater, they cause more problems than any other system. If it doesn't have leaks, you just haven't found them yet.
Look for the most recent roof you can find, Sno-Cote is definitely a good re-roof though if it needed to be done, which isn't that uncommon on used trailers. They flex and sway, which will necessarily open up a seam or two eventually. Avoid crappy silver-asphalt patches.
and if you like continuous hot water, get a Tiny Titan... http://www.americanwaterheater.com/prod ... /lpe70.pdf
(not terribly expensive) and set it to low, feeding pre-heated water into your gas heater. It cuts down on the propane use, and will recover fast enough for a 20-min shower or more when your trailer is plugged into campground power. I did it in mine, it was like gold.