Grit dog wrote:
It’s amazing, some folks’ level of conviction against extra fuel....too funny!
I’ve often wondered about the same thing.
I’ll just give it my “broad-brush” treatment and put it this way: They’re either too cheap (maybe frugal sounds better :W ) to buy one themselves, or they’re too stubborn (ultra practical? :W :W ) to admit that there ARE valid reasons besides just being able to buy large quantities of fuel at the cheapest price to having an extra/larger tanks. ROI isn’t even part of the equation for some of us.
When I had a ‘95 F350 with dual tanks, I replaced the rear tank with a 40 gallon tank, and carried the spare on the rear bumper of the camper. I never had to worry about running out of fuel, or where the next fuel station was.

I got used to that, and didn’t want to give it up when I bought my first F450. The 19.5 tire and wheel is much heavier though, so I could no longer carry it there. The OEM tank is 36 gallons though, so as long as I could have a Jerry can of fuel with me, I’d be fine with that.

I had been thinking about a way to re-purpose the spare tire carrier that had gone unused for a few years, and finally came up with a way to convert it into a fuel can carrier. The 20L can is a Swiss Army surplus, and the 5L can is brand new, and they both use the same spout.
I’ve used the Diesel can more than once on a trip when the terrain or the wind blew my expected fuel mileage and it became obvious we were going to come up short from our planned fuel stop. I’ve been glad I had it easily accessible several times.
I am a notorious pack-rat, and salvage all kinds of things (junk) that would otherwise be thrown away at work, and I had several heavy steel angle plates, a 4'x4' sheet of 1/2" high density polyurethane, a bunch of 6" long carriage bolts, etc. I was actively trying to find a way to use some of this good junk, so I could free up some shelf space in my storage building for newer, gooder junk. I bought the Jerry can holder from a eBay auction at a good price, and I bought the nylon locking nuts. Everything else is salvaged. Actually, I consider the spare tire mount to be salvage as well, since it was originally on an old truck I used to own before the F350.
Here's a better view of the contraption with the Jerry cans removed.

Backside showing salvaged 90 deg. plates.

It can even still fold down, so that it’s easier to pull the can out. After doing some major damage to both my shoulders (torn rotator cuff, torn deltoid, torn biceps), that fold-down feature has been a shoulder-saver. I wouldn't be able to get a full 20L can of Diesel out of the holder if I had to lift it straight up.

:):)