depending on ones skill level, or how much budget they have, can make a huge difference in how its done and much it costs to do it...
some tow vehicles can have the OEM fuel tank replaced with a higher capacity tank....
I have a 60gal aux fuel tank in the bed, that i have plumbed directly into the fuel tank, via a welded in fitting in its filler tube.... the main tank holds about 30 gal.... but in use, its just the same as having a single 90 gallon tank, and the entire 90 gallons can be drawn without anything more than having the tanks full....
I have it equipped with a fuel transfer pump so that if traveling companions are getting low on fuel, there is no need to worry too much about finding a fuel station immediately, as I can easily pass some fuel to them if necessary.....
the only thing I would do different in another install like this, would be to enlarge the gravity transfer line from the aux tank to the filler neck, so that when the main tank is low, I could simply fill the entire tank system from the aux tank filler neck...
as it is, the 3/8" line that I used causes a need to fill the main tank (If its low on fuel), then, move the nozzle to the aux tank filler, because the 3/8" line wont allow the fuel to transfer quick enough to fill the lower tank, as the upper tank fills up...
but its nice to be able to have around a 1000mi range without needing to stop and fill up.... especially when passing thru california....
I have side rail tool boxes on my Ram, but on my previous tow rig, I had a tank/tool box combo, which was great