In our 2015 Tacoma Acess Cab TRD Off Road: We’re towing a Forrest River hard sided pop up weighing in at 3500 wet. I’ve got a couple thousand pounds (maybe less) in the truck as payload too. It’s a mild but expensive build with All Pro Expedition springs, 3” Stage 7 Icon full system with tubular arms, 33” tires, ARE topper, Yakima bars on top, bedslide, some custom cab boxes, tools and gear storage in bed, dual battery system and steel winch bumper up front and steel bumper rear. Etc.
Mileage before and without towing..on flat ground....with my heavy foot is 16 at best. Towing it’s 12 so far. Was 14 with the RTT. This is without driving style to maximize mpg. I’m working the towing mpg higher and modifying the trailer to better balance the load. I think we can get up to the 14 towing the trailer.....with some practice. Also considering some exhaust mods which may help.