Wow, I subbed to this thread a couple days ago, and then had not looked again. I come back and this whole thing seems to have gone off the tracks !!
As to the whole idea of using one of these smaller engine trucks as "a tow vehicle", here is my take on it: right tool for the job ?
I drive a 2006 Frontier, V6 4.0L gas, six speed manual 4WD crewcab as a daily driver. This is a solid 19 to 21 mpg vehicle in daily non towing use. I pull my sub-4000 pound 19' funfinder with it, or my similar weight bumper pull horse trailer, at about 10 to 13 mpg. This truck, overall, is the "right tool for the job for me", for these uses.
We also have a F350 PS diesel dually ( "kelly's kowgirl kadillac" ) that we use for the 33' horse trailer or the equipment trailer. It too is the right tool for the job, but as a daily driver it is simply not as convenient or economic as the gas Frontier,( in spite of it using a fuel with higher BTU content per unit. ;) )
And yes, I understand all about thermal efficiency and BTU content of various fuel ( and the all important, correct stoichiometric ratio, as well as optimal compression ratio for different fuel types ).....but honestly, the bottom line is, does a given vehicle meet the needs of intended use, and can it deliver economic operation while doing so ?
I would submit that for many of the campers that folks pull, a nice V6 powered truck is a good choice. Others may need more truck, more engine, more gearing.
In an overall sense, one needs to look at the whole picture. Cost to buy, operate ( all costs, fuel, maint, taxes, insurance, etc ) and what is it's residual value at time of sale.