Two thoughts:
The Tundra is a great truck, but it is still a ½ ton truck. You also have a fairly short 5er. BUT have you verified the truck can SAFELY carry that much pin wt and you are within truck GVWR and GCWR. Very few 5er are towable by ½ ton trucks and very few ½ ton trucks can tow a 5er. The problem is heavy pin weights and limited payload. This could be your problem.
As Gary said, an inch is an inch. Before you start welding, pull the 5er onto some 2X8 "leveling" boards (both sides). Add boards until the camper is actually level, then measure the height of the 2X8's. Now you actually know for sure how much height is needed, and you can get it right the first time.