The window valances/curtain screws were stripped out of the plywood wall. The plumbing wasn't secured or "torqued" down, causing lots of play in the faucet and leaks. Check the door and window/door screens to see how well aligned they are. Lots of the little screws on the exterior were stripped or not screwed down all the way so check those.
Absolutely worst thing was the brake wiring was backwards so I had to have them rewire the entire thing, huge pain. Make sure all your lights work when hooked up and they are BRIGHT
Close/Open the slideout a few times, make sure there is no binding.
Try all the locks and make sure they operate fine, including the little shower handle lock on the back.
Most of this is just little stuff but it all adds up!