Many years ago (on a Suzuki SJ413), I had wedge tanks outside of the frame under the sills.

The top of the main tank (custom to maximize the use of space in the stock location) overflowed into the two lower aux tanks... both aux tanks (on separate switches) individually pumped back into the main tank, and auto shut off when empty.

I went from ~38L standard tank to 60L main + two 20L aux tanks, and two 20L jerrycans... yes that's a lot of extra weight, but it gave the ability to go the long outback distances between supply stops, without carrying more jerrycans (space) and the same weight.

And yes, I subsequently swapped the 1.3L petrol (gas) engine out for a VW Golf 1.5L diesel.
It was a fun little unit in both incarnations... Would like it now to save on my gas bill

[Edit] Back to RAMS; maybe you could 'T' the filler hose and run to a long tank outside the frame?

 

there is a tank switch avaliable that has 6 ports, and the electrical setup, just needs a switch and tap into the fuel lines, and the sending units, and it will switch the gauge and tanks with a toggle, just need to find a factory dodge tank with sending unit that you can mount somewhere