My '03 F-350 V-10 gas SRW gets around 12-13 mpg not towing, and about 10-11 towing the 11,000 lb '04 Designer 33' 5er. It doesn't seem to care much if towing or not - gets almost the same either way.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.