Adding to this thread...
Having shown my truck and camper to a nice couple from Holland yesterday, I got to thinking (again) about what my next truck/camper combo would be. I had been thinking of a Colorado/Canyon but discovered to my dismay that their version of a long-bed isn't - it is only 74" (6 feet 2"). Used trucks access cab (suicide rear doors) with an 8' bed are rare around here, so I started digging, looking at stuff like turning circle, payload and MPG, and I found some cool stuff!
First, using the handy google search phrase - site:www.wanderthewest.com mpg I found these previous threads:
http://www.wanderthewest.com/forum/topic/6082-an-experiment-how-much-does-the-camper-affect-mileage/
http://www.wanderthewest.com/forum/topic/12966-comparing-34-ton-trucks-fuel-economy/
http://www.wanderthewest.com/forum/topic/3979-what-is-your-mpg-with-fwc/
http://www.wanderthewest.com/forum/topic/1194-vehicle-type-and-fuel-mileage/
http://www.wanderthewest.com/forum/topic/12397-mpg-report/
http://www.wanderthewest.com/forum/topic/2742-expected-gas-mileage/
http://www.wanderthewest.com/forum/topic/9160-1st-gen-tundraeagle-mpg/
http://www.wanderthewest.com/forum/topic/1072-gained-2-mpg/
http://www.wanderthewest.com/forum/topic/10593-anyone-using-ram-ecodiesel/
http://www.wanderthewest.com/forum/topic/10836-cummins-67l-diesel-vs-gas-64l-hemi-v8/
Further, I note that there is very little consistency in the threads above about loads, speeds driven, wind direction, elevation changes, how aggressively we drive, you name it.
Plus I love my 2002 GMC 2500HD with the 8.1L (496ci) v8 and Alison transmission... It handles the load of the Hawk very well. Well enough that I can still drive speed limit everywhere, and even (what others might find aggressively) up hills or in the corners do I slow down. And I get 11.1 mpg (US) average.
I used to think I would get much better with a newer truck, or with a diesel. And even if I did, I decided a while back that even with a significant uplift in MPG, the cost of fuel vs the cost of the uplift to a newer truck, it just isn't worth it to upgrade. BUT, my truck is a short 6.5" bed, and I want a long bed truck. So an change is in my future regardless.
That upgrade will be a 3/4 or 1 ton truck, as I have no interest in running overloaded in a 1/2 ton (maybe an aluminum Ford F-150 though). Finding MPG info for 3/4 ton trucks is tough, as the EPA doesn't measure work trucks. Then I found Fuely:
http://www.fuelly.com/car/chevrolet/silverado_2500_hd?engineconfig_id=64&bodytype_id=11&submodel_id=
This is my truck, and I feel like these numbers are very real compared to mine.
Then there are these guys who do real world testing:
https://www.tfltruck.com/2017/09/heavy-duty-pickup-truck-towing-fuel-economy-trailering/
https://www.sdtrucksprings.com/best-mpg-while-towing-with-half-ton-pickup-f150-silverad-ram-1500
https://www.consumerreports.org/pickup-trucks/heavy-duty-pickup-truck-fuel-economy/
https://www.tfltruck.com/2018/03/top-10-most-efficient-trucks-towing-a-7000-lbs-trailer-on-a-100-mile-highway-loop/
Testing rigorously at 70 MPH and a 22K trailer, they are getting 6.5 to 8.6 MPG with the big diesels and gas V8's.
So I discovered that 11 mpg is actually really good, especially since these guys are doing 70 mph, and I get 11 ish at 80 mph. Mind you, that's with a camper on the back, not a 22K pound trailer. From what I can glean from the posts above, a trailer impacts MPG about 50%, and a FWC about 20%. In my case, the load seems to matter less, with less than a 10% difference between loaded and unloaded.
I also see from TFL Truck that diesels do NOT get better MPG than gas engines when fully loaded and working hard
https://www.tfltruck.com/2018-gold-hitch-best-towing-off-road-trucks/
Hope this helps others make a decision, and I would love "advice" on a 3/4" access cab long bed truck!
Vic