@llamalander provided a great write-up on how a propane fired on-demand hot water set-up works.
It is exactly what I have on my trailer, though attached, not portable. The armored black box at right rear inside of my trailer in the image below is an on-demand propane-fired water heater. It draws cold water from a 22 gal tank underneath the trailer via a 12v pump and sends heated water to two quick-connect locations; one under a rear corner for a faucet/sink, and another under a front corner to which I can attach a shower hose.
I extended my shower hose with an extra length so I have sixteen feet. That way I can run it into a privacy enclosure, or use it to wash gear more easily, etc.
Most small on-demand water heaters like this are electronic-ignition powered by a couple D cell batteries, so you won't need to worry about figuring how to wire it from a 12v power center.
I love mine. It has proven to be simple, easy to maintain, and problem free in the couple years I've used it, and I use it a lot. When I know it's going to get below freezing, I drain the system and draw RV anti-freeze through the lines and heater. At least once a year, I run a bleach solution through everything, including the water tank, heater, plumbing lines, and any containers and hoses I use for filling or around camp. See insta post below with the details on that.
I haven't felt the need for it to be portable or in a Pelican box. If I ever get rid of the trailer, or start using my van only for trips and not base camping with the trailer as much, I'll set up the same thing in the van, mounted on the inside of the back door or in a vented bumper box.
..
Let me know if there are any other questions I can try to answer.
Good luck, hope it works out well for you.
.
Roaddude - Traveling Photographer/Writer/Artist On the Road In North America. Gear, reviews, people, places, and culture.
roaddude.com