I've been towing with my 4-Runner V8 for the last two years. The TT is 2800 lbs empty and 3200 lbs loaded. Yes, I travel light, usually just myself or with one other. TT was weighed at the scales. I also weighed the hitch weight with my sher-line scale - 500 lbs. with full propane and battery.
I spent over two weeks in the Rockies in Colorado a year and a half ago. The truck did an admiral job. It obviously slowed down with the grades and higher altitude/thinner air, but it downshifted fine, usually, and when it didn't, I did it manually. The engine is smooth and silky, so it wasn't bad. The platform was also stable for me. I thought this might be a problem, but it was fine.
To put this in perspective, I have had two larger TTs and have towed with a 1/2 ton SUV and a 3/4 ton SUV, TTs were about 6000 and 7800 lbs loaded. The 2500 Yukon XL 8.1 towed the largest TT very well as expected. However, RELATIVELY SPEAKING, the combination I have now tows ALMOST as well. With that being said, I would NOT want to tow too much more (maybe up to 3600-4000 lbs?). That is just MY preference.
Regarding payload, again this isn't too much of an issue for me. I don't remember the exact payload rating, but I think it's in the 1100-1200 lb range. Heck, this isn't too far off some 1/2 ton pick up trucks. Yes, I have 500 lb total hitch weight, so I usually keep my generator/inverter and firewood (cut up 2x4s) in the TT. Obviously, this combo is not good for a family of 4 or more, because you'd be pushing the payload limit too much IMHO.
It seems no matter what TV/TT combo I have used, I get about 7.5 - 8 mpg, same as the present combo with the 4-Runner and Sportsmen Classic. Like everyone says, the wind resistance from a TT is your worst enemy. YMMV.
Feel free to PM me if you have more questions.
'14 Sportsmen Classic 190
'17 Jeep Grand Cherokee Trailhawk 5.7 Hemi