Another thing that kills me is how they ram sharp-pointed screws in all over the place without regard for wiring that is in the walls or ceiling. While doing various mods on 3 TTs now, I've found lots of wires pressed up against the tips of screws. I've lost a lot of blood from scratches on my hands from the dang screws when reaching blind into spaces to do wiring mods.
I've also found lots of twist-on marrette connectors poorly tightened. When they run wiring in the walls and ceilings, the insulation often gets compromised from getting compressed and voids being created. Sometimes, there isn't any insulation anyway because there were too many wires in one area.
We have a brand spanking new TT. Took the cover off a dinette seat to install an EMS. Lo and behold, there's a PEX water line in there that is kinked at 90 degrees and collapsed just before it goes under the floor. I think it actually does a 180 turn to get to the water pump but can't see underneath. They saved a few minutes of additional work and a couple of fittings - nice... Not sure how they will fix that other than to take the underbelly off. I guess it gives the dealer a chance to make some money.
I hesitate to think it what is lurking behind all the enclosed underbellies out there. Maybe that's one reason they're so common now. They get to hide all manner of sins... It sure would be interesting to go to a plant and watch them build a unit from start to finish.