In my searching for a decent used TT, many of the slideouts I looked at had issues, with either seals, or mechanisms that were out of alignment. With a slide out, all it takes is deploying one time when the trailer is not leveled properly to break something. Every one I saw that had an issue appeared to have been slid out with a crooked trailer. There was one I looked at that for the life of me could not figure out how they could have twisted the frame so much. The bottom was a good 4" out on the one side. How it didn't get bound up when retracting, I will never know. They look like they are great for adding room, but also look like they would be a maintenance nightmare when damaged.
I settled on a Coleman (Dutchman) without slides, and quality is, lets say, adequate, you get what you pay for. Sacrifices have to be made in order to keep weight down, and I can see many many areas that could use improvements, but they would add both cost and weight. I had looked, and tried to buy, a 2003 KZ with the two slideouts, and quality was OK, the floor was soft, and one side of the slideout on the side had interior damage, probably from being hit while retracted, the one side was knocked off, but aside from that, everything else appeared to be in pretty good shape considering it was 13 years old. I just hope the Coleman looks that good in 13 years.
I also looked at a newer KZ (Keystone Outback) A 2011 250RS model of the same design, and quality there was about the same, saw no damage or worn items inside, but could easily tell it was stored outside as many of the plastic parts were sun stroked hard. From what I saw in those KZ, if you can afford one, I would look at those, they seem to be well made, I just couldn't afford the second one I looked at, I wanted it, but it was well above my price range. IT is still at the dealer, unsold as I type this too. Sunnyside RV if you want to see it.