I found the same with my Dodge Cummins. I have a heavy steel deck and had to carry a dead VW Jetta from Winnepeg to Edmonton. When I was loaded the rear was down a bit, even on HD rear springs. About 90psi in the air overloads put it just right and it rode fine.
1988 ChevyVan30 with "Escaper" camper mod,
1979 35' 5th Wheel "Cobra" travel trailer,
1993 Dodge Cummins Turbo Diesel,
1980 Honda GoldWing,
Trend: "all old like me! "