I am also going to jump in and say AH...AH...Ahhh....BU!!$4/T.
There is too much aerodynamic drag with a trailer for a tailgate to make any difference in towing mpg.
He is trying to justify the purchase. LET HIM! It is worth it for convenience.
If I am perfectly straight, I can lower my stock tailgate, but I almost took mine out two different times. I CAN'T screw up towing, because I drove semis for 17 years, and the DW thinks I can back up anything anywhere, and I pretty much can, but, EVERYONE screws up. That is just part of being human.