I put a plate heat exchanger under the hood and use a demand pump (boat pump) to draw from the ~11 gallon stainless tank I had made behind my left rear wheel. (It fills from inside the truck to avoid surprises) the pump draws through a sediment filter (I broke one freezing it... oops) and then to a tee- one side goes to the heat exchanger, the other to the “blue” hose connection on the side of the truck. From the heat exchanger, it goes to the “red” hose connection.
I took a home shower control and plumbed it with red and blue washing machine supply hoses. A braided hose goes up to the shower head, with a couple PVC hangers attached do I can hang it on the roof rack or roll bar.
Stand under shower and adjust water temp as desired. The hot is DAMN hot- like make tea hot. It’s quite an indulgence but makes the worst travel day much better. (Burbon helps too)
I have a stainless tube in the tank for pick-up. I placed it so that it is accessible from the (1/2” NPT) drain plug, so if it gets really funky I can run a bottle brush up there. Vent line for the tank goes to the top of the interior. It will probably leak if I roll the truck onto the left side, so I try to avoid that, so I don’t get the interior wet...