I'm having same issues just purchased 2020 1500 lt Duramax. Pulling a 2009 jayco 26bh
completely bottoms out the suspension with the truck empty. Bought a 10000lb weight distribution hitch and having exact same issues. Remeasured everything 3 times tried setting hitch up for Max weight 1000lb tongue and it still can't compensate for the excessive sag. My 2016 1500 didn't have any issues at all nor the newer f150 the previous camper owner was using. I really really like this truck but there's definitely something wrong with the rear suspension.
2009 Jay Flight Floorplan 26BH
Weights
Unloaded Vehicle Weight (lbs.)
4,445
Dry Hitch Weight (lbs.)
580
Gross Vehicle Weight Rating (lbs.)
7,500
Cargo Carrying Capacity (lbs.)
3,055