With our '08 F250 with a 350 gas we average ~16.5 highway (doing speed limit) single; 14 city; 12 towing our '86 Avion 34V. towing will dip down to 10 in extreme hills/foothills, 8-9 mountains. Extremely flat we have even averaged better than 13! Speed of ~55 when towing. When we had an SOB average was 8-9, best was 10, and worst was 6.5! And the SOB weighed 3/4 of what the Avion does!
I wanted to add that back in '03 a friend had done some research that seemed pretty reliable that you could figure a payback on the extra cost of the diesel (original cost, fuel, & maint.) once you went past 81,000 miles. Don't know what the figures would be today.