Good question. You'll find a wide variety of answers. It really depends on how and where you drive, what you've done to your suspension, how much stuff you carry, and whether your FWC will be on full time or part time. As I recall, there was a good discussion about campers and mpg on the Expedition Portal, plus many related discussions here.
I have a 2016 Tacoma (SR5, V6, automatic, 4x4, access cab, stock except for e-rated stock size Cooper Discoverer AT3 tires, RideRite airbags, Rancho adjustable shocks/struts). I carry a 2016 Fleet (front dinette with typical kitchen, furnace, hot water, 200 watt flexible solar panels on roof; dry weight approx. 1320#). My camper is on the truck 5 months out of the year and 755 of the time I'm by myself. I try to pack as lightly and simply as possible (think backpacking with a truck/camper), but I estimate I'm still typically 500 lbs. over GVW. Airbags are typically inflated 45/25 lbs (left/right); tires 45/55 lbs (front/back), shocks are set 7 front and 8 rear. I adjust tires, airbags, shocks depending on road conditions, handling and how much weight I'm carrying. It took me a while to learn what combos work best in what conditions. If you search the WTW truck forum you'll find many lively discussions about suspension modifications and handling issues. I have 24,000 miles on this combo (only 8000 miles with adjustable shocks, which I find are essential for my needs).
I drive very conservatively and because of this, generally avoid high speed highways. The majority of miles are paved roads at 45-60 mph, but I log 2500-3500 miles per summer on high-speed (well-graded) and low speed (wash board, pot-holed, rocky, sandy) FS and BLM roads. I occasionally travel on 4-wheel drive roads, but I don't seek them out. I consistently average 20+ mpg (19-22 mpg) across all conditions. In my experience, driving above 65 mph and/or driving into headwinds are mpg killers and can easily drop mpg's down into the 15-17 mph range. The truck can handle mountain passes (4-7+% grades), but I drop my speed to 40-50 mph, but I could push it to 55+ mph if I had to. If I'm conservative, my truck/camper combo easily carries the load and handles well. It drives best with ECT engaged and it doesn't seem to change my mpg. I often use the manual shifting mode to ascend/descend big mountain passes. If I'm holding up traffic, even one vehicle, I pull over or otherwise make it easy to be passed. I have the retired luxury of not being in a hurry but I know other folks sometimes are.
Hope this helps. Good luck with your research and decision!