I have a 2017 GMC 3500 4 wd reg cab srw with 6.0, 5th wheel package.
It weighs 7000# with myself and a full tank of gas, 3000# on the rear axle. The tires are 18" which gives me around 4000# to put on the rear axle. There are tires that can raise that value also.
Brand new we pulled a gooseneck trailer with van on top which weighed around 10k#, pulled 70 mph easily, got 7 mpg and that is break in mpg avg unloaded/loaded.
I have gotten a high of 15.1 mpg mostly highway and around 13 mixed so far unloaded, 3000 miles so far. Key to this is getting your alignment checked and spec no more then .060 in total, My machine wouldn't do the large tires so I stringed it for straight ahead with maybe a little toe in, you can tell the difference driving while new when everything is still tight. A truck this heavy with big tires is going to be sensitive to alignment and how it effects mpg, coming off the assy line they set them pretty coarse. The 7 mpg was factory alignment and it looked like it was a good 3/16".
Getting the engine broke in quick is a key also, after the trailer pull I drained the oil and watched the glitter come out with the oil.
I have the Bigfoot 10.4 I am getting ready to load, so I'll see how this works out. I also put a Hellwig sway bar on the back and the truck has the snow plow prep package.
I really like the tight turning radius of the reg cab long bed chevrolet.
Another thing, if you have the 5th wheel prep package or aftermarket, you can make a plug to insert into the 5th wheel hole that extends above the bed enough to keep the plywood sheet from moving. I made a plug out of hedge on a lathe then center your plywood sheet and put a hole through it.
And you know what, if things get a little out of hand hauling the camper, I'll just go a little slower. I know, hard to believe.