After debating the different ways to heat my water (propane instant, electric instant, heat exchange, separate tank with electric coil, solar tank, etc...) I decided to go with the least invasive/cheapest way for the time being. I decided to just add a 12 volt heater element to my fresh tank and temperature control it to 90 degrees, thus preventing the need to mix with cool water. My fresh tank is a 20 gallon medium density polyethylene tank and it located inside my vehicle in the rear.
Parts I needed for this job:
-Teflon Tape
-12 Gauge Wire
Started off with removing the 1-1/2" plug in the rear of my tank and installing the heater coil directly into the fresh tank using Teflon tape.
Filled the tank up and checked for leaks. Once it was good, I started into the electrical.
First I wired my temperature control directly into my 12v supply (fused from house battery). Then that output (triggered by temperature probe and settings) went to a 12v switch, which in turn triggered the 30/40 amp relay located in the rear by the tank. The relay then output the 12v via the 12 gauge wire directly to the coil. I did it in the manor to have the high voltage line be as short as possible.
I installed the switch to turn on the heater element on the dash. I did this so I can turn it on while driving. The red light will illuminate when the heater is turned on by the switch and the light will go out once the water hits the desired temp (90 degrees) and the thermostat control turns off the supply to the switch. This is nice so you know when the water tank is up to temperature.
I used some insulation to isolate the temperature probe from the outside air. I made the decision not to drill another hole in the tank for the temperature probe and instead I just tapped it to the outside. That may change later, but from my test the temperature difference was less than 1 degree from inside the tank and outside.
The instructions for the temperature controller were fairly straight forward. I set it at heat, 90 degrees with 10 degree parameter. Meaning when on it will heat to 90 degrees and then not turn on again until the water hits 80 degrees.
My concern with a 12 volt coil is it will take a long time to heat up the water and at 300 watts its going to draw a lot of power (25 amps). This puts a huge strain on the electrical system and it why many people go with a propane heater. I do not need super hot water, I would just like it to not be super cold for showers. I also drive a lot between spots and since I most likely will only be able to run the heater when the engine is going it should be ok for me. This is not a system that would work without the engine running or some external power source in my opinion.
Here are the other methods to heat water I considered and why I didn't go with them:
-Coolant heat exchange - to much work and would require running coolant lines to the rear of the vehicle, more chance for failure/leaks.
-Propane Instant HWH - Needs to be vented outside, none seemed reliable in a low price point
-Electric Instant HWH - To much power demand, my system could not keep up
-Separate Smaller Hot Water Tank w/coil - Required mixing of water (cold/hot) and running multiple lines and additional tank.
-Roof mounted solar tank - Required a lot of piping and being able to fill on the roof. Would require mixing of cold water as there is no temp control.
If anyone has a better idea for hot water I would love to hear it, this is something I have debated for a while and my camper van is a constantly evolving project.
Update (10/2018): Did a camping trip in Copper Harbor, MI with this setup and it worked great. Took about 3-5 hours to heat up the water, but everything worked as it should and as long as the engine was running or the charger was plugged in then it had plenty of power. It can run without AC input or engine, but it will drain the batteries down a bit. Was really nice to have the hot water and this was the cheapest/easiest way to do it.
Update (06/2020): Still using this method, but it does take a really long time to heat up. Looking at changing the system either to a smaller tank or trying out a propane instant hot water heater. Either way I go I will keep the old system in place, it does not take up space and can still be used on long road trips.