Your friend wants more HP, and better fuel consumption ? Those two don't go together.
I've had a 2014 tradesman 2500 CTD 6.7 4X4,3.42 gears with the 68 rfe and towed a 14,000 lb Montana 5th wheel , and never needed more power to pull any grade out West on a highway system to do the posted speed limit. My current pickup 2015 (3500 ctd Dually,4.10 Aisin 4x4 megacab) pulled a 18,000 lb Landmark 5th wheel through all lower 48 US states, and never needed more power to do the speed limit. 30,000 miles on the road and never needed any after market stuff.
You didn't mention how big this camper is. It really doesn't matter, because that 3500 should be able to accommodate a very heavy camper, and still have plenty of reliable stock power to do it. If the truck has the gear select button on the shift lever, tell your friend to use that like a manual transmission. He can control how long to stay in say 1st, 2nd , and 3rd gear to get it moving faster. I use mine occasionally. Not related to HP, but the only thing that 3500 might need is more rear suspension, like air bags.