Who said it was a fiver? I've posted this before. A number of times.
My FE numbers are consistent with more than a dozen other second and mainly third gen CTD owners pulling 7-10k lb, 28-34', all-aluminum aero travel trailers. Who also live and travel in the South Central US. 55-62/mph, most of them.
It's also at 60% of solo hwy mpg. Fits the model. 1967 or 2017: towing mpg ((all else the same) is mainly an aero problem.
Any of you all taken the TV out for a stroll at same scale weight less hitch pounds down same roads st same speed? It's instructive.
I started from scratch for spec as longest life and lowest operating cost were important. Then worked the details of achieving and maintaining low fuel burn as part of that. FE is just a marker, like brake or tire life (which is over 100k with this truck). It's part of cost prediction. Budgeting.
It ain't rocket science.
I've also posted -- more than once -- how due to a challenge I upped my town mpg from an already above average number by 20%. No loss of convenience, or stunt driving. At the then current price of $4/-gl for diesel, and extrapolated out over a year, this would underwrite 5000-miles of free vacation fuel.
Think anybody ever took me up on it? Ha! Today's Americans seem to have no interest in disciplining themselves when it comes to vehicle fuel use. Even if it paid nearly half the travel expenses of an RV vacation.
Keep records. See where it isn't painful to change. Establish new habits. Etc.
Here's a clue: Average MPH. One man tows at 61 and another tows at 68 with all else the same. Which gets to the destination faster? How much?
Believe me that I'll knock holes in assumptions right and left. Time versus fuel, etc.
"Average mph", with number of (as well as time control) of stops on a 300-mile day will rapidly close the time gap. The average is NOT the travel speed. It's far lower.
Lane-changes, use of brakes, and lack of cruise control use rapidly increases fuel burn.
The gap between those two drivers as to time is smaller than it appears. Far smaller. Fuel burn, however, is significantly weighted against the "faster" driver in typical east of the Mississippi driving.
The case is easily made that on a typical RVer travel day, a few extra minutes with the TV channel changer doesn't warrant the higher speed. Not when reduced fuel burn over the average annual 5000-miles can then cover other expenses.
That it's a speed above safe pickup truck operation is another consideration.
Etc.
The info is out there for those interested.
FWIW, I bought both truck and trailer for a little under $30k combined. A long lived pickup and a trailer with an indefinite lifespan can be had more cheaply than supposed.
Again, etc.
If both camping and cost control are important, devote some time to it. Internet makes that easy.