The load limiting factor on a pickup is the tires. Easier to increase the payload capacity of a factory stock truck by getting the DRW option and for a greater than 4,000 lb. payload it is the simplest approach.
An alternative is 19.5 rims and tires which provide 8800 lbs. of support at the rear axle. Even with E rated tires there are ones from Toyo and Nitto that provide 7500 lbs. of support with the factory rims.
The 855 and 855s are as light a camper as you will find in that size and easily managed with a SRW truck. Regardless of the truck you are probably going to upgrade the shocks and add a heavy duty rear anti-sway bar (Ford especially).
You are correct in going with a 4.10 rear end gear set. It will provide for faster acceleration with the camper load and a gas engine. The difference in MPG will be negligible with today's 6-speed transmissions that have 2 overdrive gears.
