I have 2- 55 gal L tanks in bed of truck with a slider hitch. If you dont have a slider you still might miss them. It will be close with the long box but you might clear them with a standard hitch. Depends on 5th's shape on front. Short beds wont work with these tanks but they have other shapes that will fit below the bed.
You might get just one 50 gal square tank.
There from Northern Tool and Equipment. One tank holds 55 gallons of fuel for car. With a shut off and rubber hose comeing out under truck to fill gas cans. (no pump).
The left tank is deisel fuel piped into my main tank. They also sell a kit for this that shuts Aux tank off when main tank is full. Just a check ball in the pipe.
I can run from Prescott, Az With a D-Max and 40 ft 5th loaded to Glamis and home and still have a 1/2 a tank left.
I never run out of fuel for the car either.(but its a Honda)( Its still full from last weekend) Car ran like crap)