O gawd, frame quality? Don't get me started... It's shameful what many of the TT manufacturers do.
Before buying, look at the axle rating and load capacity rating of the tires. Some manufacturers load them right up to their ratings. Take 12-13 percent off the GVWR for the tongue weight removed from the axles. (Tongue weight can be up near 15 percent sometimes tho.) You want at least 10-15 percent reserve load capacity in the tires and more is better. In some cases tires are loaded right up and the manufacturer has not accounted for a TT weighing more on one side due to slides, kitchens, etc. Maxed out tires invites blowouts and maxed out axles invites bent axles. Some manufacturers offer optional tires with higher load rating.
Avoid buying a TT that has a frame made from 3 pieces of sheet steel welded together and *look* like like a regular I-beam. They flex a LOT and can cause serious issues including damage to the superstructure like aluminum framing in the photo. We took a TT with the flexible frame to a frame & axle shop with in days from buying it new and they said it was the worst frame they had EVER seen. They said much of the welding was very substandard.
Get a TT that has a higher CCC rating. Some are down around 1500 lbs and some are up around 3000 lbs. Some CCC figures cannot be trusted and actual dry weight can be higher than the factory dry weight figure. Ours is 190 lbs higher than what the sticker says it was when it left the factory. Low CCC can sometimes result in overloading. If we were to travel with one full holding tank, we'd be over the GVWR.
Brakes, brake wiring, bearings, bushings and even the springs can be a problem. Frames & components have a minimal coating of paint and if your TT was delivered in the winter with salt on the road, you can end up with rust everywhere. The enclosed underbellies are mostly there to hide bad workmanship (being facetious but there is some truth to this). Holding tanks can be poorly secured to the frame and on occasion will drop out. "Heat underbelly"? Want your furnace to heat the great outdoors? They just run a duct or two under the frame without a return.
Sometimes the welding on frames is so bad that they break in a major way. The 2nd photo is a forum member's TT and the A-frame broke clear off and he had to get a welder to fix it.
IMO, one big contributing factor is the lack of any regulations for frames & suspensions. Manufacturers like Lippert, who own probably around 95+ percent of the market, can and do build frames however they want without repercussion. If there's a problem, they'll say things like "owner overloaded the trailer" or "it's within specs".
Outdoors RV, Nash & Artic Fox use the same in-house built, off-road certified, heavy duty frame. A few manufacturers like Jayco & Lance use the BAL/Norco
"Ultra Frame" or
"NGX" frame which has no welding on it and uses a type of rivet instead. If we were to ever buy another TT, I'd be looking at any without a Lippert frame like these.