I did not follow your post so well, but will chime in.
With a light camper like that then just a little unbalancing of side ot side weight can make it lisp. Ever seen a fat guy in a small car? It will lisp from his extra pounds.
Too often RVs are built out of balance with all the stuff on one side. My last trailer had the fridge, stove/oven, water heater, sink and electric stuff all on one side. The other was a dinnet (light and lots of open air), space heater (way light) and a door (more open space). Though it did not lisp.
Be sure to put your stuff on the side you think is the lighter one.
Also, could be your old truck shows sagged springs when loaded a little that it does not when not.
You say the bed is level? If not then it would not be obvious unloaded, but with a tall thing it would be more obvious.
If you have air bags and they leak, then I would not look further.
Does it lisp enough to be a driving hazzard or just an annoyance that you know it lisp and that bothers you?
Old stuff sometimes you just go with it and don't complain too much, kinda like us old folks.
Sounds like a simple glorified camper shell, maybe low-priced to begin with, possibly not the greatest attention to detail. I have a cargo trailer that has the left wall higher than the right. It is obvious to me looking at the rear door as the roof slopes up to the right. I can't fix that.
Turnbuckles… Does it lisp with them off or loose? It is possible to tighten the first one too tight and that makes it lisp then cannot be corrected by tightening the other side.
