You can research until the cows come home and find out that every brand is the absolute best and the absolute worst.
I know that is what I found out over the past six months before recently buying my Flagstaff 5er. Wife and I decided on this unit because it was priced exceptionally well and was in as near new condition as a 7 year old unit could be. I wish I could provide more detailed information regarding specific brands but honestly I think the best way to find out is to actually walk through the unit see how it has held up. Visit multiple dealers and walk through their used units, even units you arent interested in, and see how brand A compares to brand B and C, etc. Look in the areas that really get used like the kitchen and bathroom. Check out the slides. Check the walls and ceiling for any water damage. Best of luck in your search!