Looks like you have checked out the trailer, but what about the truck? On a vehicle as new as yours with few miles, it would seem to be unlikely that there would be a suspension problem. But ya never know. That would be my next step.
If you can find an empty parking lot somewhere, maybe you can put the truck through some slalom type turns at a speed that would challenge the suspension and see what happens.
Have you scaled your rig? Maybe you have too much weight to the rear.
That’s all I got…………..
2014 Keystone Laredo 292RL
2013 Palomino Maverick 2902
2018 GMC 3500HD, 4x4, 6.5' box, SRW, Denali, Duramax, Andersen
DeeBee, JayBee, and Jed the Black Lab
The hurrier I go the behinder I get. (Lewis Carroll)