We have a 3/4 ton Chevy gasser to haul our 32' TT TH around the four corner states and mountain west region of the country where we live and RV most often, there is a difference in mpg when our 2 freshwater tanks and both fuel tanks for generator and fuel station are full, we don't always bring toys along.
I would think the difference would be even smaller for a diesel tow vehicle, depending on what type of RVing we plan to do our rig can vary in weight by over 2K#. We always carry at least one tank full of fresh water even when going where water is available, our TT TH is 7.5K# dry, 10K# wet with toys in rear.