I didn't read this entire thread barely scrubbed the first post. I'd like to share some thoughts, if I may. While out camping out of the back of a landcruiser, water is scarce where I go, which is usually NM and UT. We all have to carry our water.
We just got back from camping for two nights at the Devil's Kitchen campground inside the Needles district and one night at Manti-Sal mountains. Yes, it was brutally hot and we REALLY needed a nightly shower so that we can fall asleep in the 70deg night time temps.
Following the KISS principle, I have a simple shower bag for both of us. When we move daily, the sun doesn't get to warm up the shower bag so I'll add one large jetboil of hot water into the shower bag which has room temp cold water. Funny enough, on our last trip, the air temp in the vehicles must've been around 150deg because that shower water was very warm! We didn't bother heating up the shower water during those hot days. The last night's camp at 8,000' was divine and the shower water was STILL hot. I carry a water bladder that lives in the footwell and probably have approx 10gal in it. A water bladder is better than a water tank because it implodes and makes less whoosh sounds as you drive the rig.
I used to have a Helton heat exchanger deal under the hood but I really got tired of dealing with the hoses, shower head, pump and all that BS. I used to toss the pump and the shower head into a 5gal jerry can then recirc the water while the engine was either idling was shut down after a long drive. This simply wasted too much water for my taste and the setup time was too long. Room is precious when you're camping with a dog in the cargo and I had to simply and downsize.
However, when I'm using the camper, I use the Ecotemp 5L for a more luxurious shower. I did drill a 1/8" diam hole into a penny, then use it as a flow restrictor for the shower wand. This brought down the water usage to an acceptable level. In order to avoid getting a "cold water" sandwich, I use an anti-scalding mixing valve so the water temp is always "just right". If you don't use this device, you end up wasting water because you get cold water, then hot water, then you fiddle with knobs, then you get "just right" water. Meanwhile you're running water into the ground which is unacceptable where we camp.
This weekend we're going to a large reservoir and we can't wait to have natural water all around us. Still, no shade trees anywhere but we have our own shade.
I hope I helped!