Wheel wells are not a good place to take a measurement. The sheet metal covering the flooring, from underneath, is very thin and are distorted.
Park on a known level hard surface and measure from the axle mounting spacer to the surface.
Get it on a level ground and measure the frame on both sides front and back of the rig. You should have the same measurement on both sides in front and back. It is the side to side comparison to use not the front to back. You could have a warped frame. My dealer is going to measure my 2604 when I take it in next as I have the same issue. I use the rear window defroster wires to check that the trailer tires are all OK and I noticed the trailer was not level with the truck as the last trailer I had was.