This is not a new mod. In fact, it is a replacement for the original that I did back in 2006(?) I think.
When I was in Africa, I had a black plastic water can on the back door, and by the end of the day, I had five gallons of hot water for two showers and two shaves (this was 2001).
But Mike had a (POS) coper tube heat exchanger for his shower, and I was originally planning to remake one of those, as the purchase price was disproportionate for what it was (and then his failed), so I did the right thing and completely over-engineered the thing.
Got a stainless-steel plate heat exchanger and mounted it under the hood. Took the old demand pump from my Series and mounted that in the back, along with a "whole house" water filter. Mounted an 11-gallon stainless tank behind the left rear wheel (that was custom as well- I made a mock-up in cardboard and a shop made it for me) that fills from the interior, to avoid anyone siphoning or contaminating my fresh water. Tubing from the pump runs forward to a tee; one side to the "cold" connection and the other running forward to the HE. Out from the HE to the "hot" connection on the side of the truck. These are standard garden hose fittings, and I use a pair of washing machine hoses that feed a standard home shower control. The shower head uses a stainless braided line (for strength) and the head attaches to this, along with PVC pipe hangers (upside down) that hook on the roll bar. This gives me a hot shower (well, and temp really) with total control at the point of use.
Just recently I had to replace the HE, as the freshwater side did not get drained completely, froze, and separated the plates on me, causing coolant to leak into the freshwater side. Found one on eBay VERY cheap and designed and fabbed up new mount brackets (aluminum this time- the steel ones were dissolved!). Still need to hook up the freshwater side, but I need to flush it out really well first after the coolant ingress. Pics below:
These were the original brackets I made... They didn't fit the new, smaller HE anyway, but they had the structural integrity of a taco shell.