2001 f150, 5.4 engine, 4x4 extended cab, 8,300 tow rating
18mpg loafing down the road empty
9-12 mpg towing
Had a 93 Landcruiser, straight 6 engine, 5,000 tow rating that towed the boat well and returned about the same mileage.
use caution when truck shopping. The so called tonnage rating of trucks means very little. There are half ton trucks that do return about 22mpg but in their high mpg form don't allow much for hauling or towing capacity. There are also 1/2 t rigs that are capable of towing and hauling more than many of the older 3/4 ton trucks. A properly equipped 1/2 ton will tow your boat with no problems.
It looks to me like with the new emissions requirements on diesel engines, the difference in mileage between the gas and diesel engines is reduced making it durn hard to justify the extra cost for the occasional tow.
It's hard to cheat physics, to get a rig that is designed to haul much of a load, it's going to be consuming more fuel
things to look for-
tow package, usually includes much more than a trailer hitch, some will have a heavier duty transmission, transmission cooler and stouter suspension
rear axle ratio, the higher the number the more you can tow but the worse mpg you'll get.....a 4:10 will tow, a 3:53 will return better mpg.....this is one means used to advertise those trucks that get the good mileage, they're geared to be just idling down the freeway but can't tow squat
GCVW, gross combined vehicle weight, that determines how much you can load up on old Betsy after subtracting the weight of the vehicle itself.
rigs...arg, I think my next truck will be like a C-Dory, slow, rough riding, tough as an anvil and a little odd, I'm thinking pre emissions Mitsubishi Fuso FG140