Get taller tires if you do a lot of hwy driving, it really helps.
265/75-16 BFG AT, 2WD, SRW, E350 Extended 7.3 PSD = up to 22.5 mpg. average about 19-20mpg hwy.
Towing a 7000lb TT, cargo inside van 1200lb and I still get 13-15 towing.
All done at posted speed limits.