Go narrower tires, and newer gassers have 2+ more gears. Though when towing, unless it's flat and not windy, you won't be in 6th much.
I know you aren't interested in a V6, but my 3.7l 14 F150 SCAB on "34s" (33s) 3.73 gears with 1000# in truck towing a 2500# pop up camper gets 17-19 l/100 or...