Let me share my experience. I tow a 7k camper with a Tundra CrewMax. We usually carry 100-200 lbs of bikes in the back max. so I am close to PayLoad.
My tundra used to do the same thing when I moved form a popup to TT.
The rear tokico`s where shot to death at 90k. Upgraded to Blistein 4600 HD rear shock. This fixed the roller coaster ride .
This winter I added some airbag I plan to run on adjust the hitch height back to where I set it up after adding people/stuff.
Again, mine used to be like riding a rollercoaster when I hit a transitions on the highway. All that is gone just with the rear shock swap.