I installed an Aluminum Tank Industries 46 gallon auxiliary tank in my truck bed. With cap, it measures 19" tall. My tonneau is a tri-fold and sits on top of the bed rails. I installed a kayak hatch in the tonneau to access the filler cap. Works perfect.
I expected the tank to pay for itself by the middle of next year by reduced cost of where I buy diesel. With the 67% increase of diesel in my area, not having to fuel on the road has already offset the tank cost. The tank was worth it from the outset due to the added convenience. Haven't fueled with the trailer attached since the tank install.