It's all about elevation changes, wind speed and direction, and especially your towing speed. The only one you can affect is towing speed.
We've done two 3,000 mile plus towing trips pulling a 25ft 5,500lb RV trailer. We averaged 11 mpg on both trips. Towing speed is 60 mph. Towing at speeds faster than 60 mph will dramatically reduce your mileage.
I don't believe that the K&N or other mods will do much, if anything, for your mileage. And the cost for the mod vs the payout will be disappointing to you unless you are doing the mods for other reasons.
2012 F150 FX4 Ecoboost, 2016 Surveyor 274BHS
2003 Yamaha FJR1300, Demco Premiere Slider
1969 John Deere 1020, 1940 Ford 9N, 1948 Ford 8N
Jonsered 535, Can of WD-40, Duct Tape
Red Green coffee mugs