Here's an interesting idea... What if you have a permanent water holding tank, water pump to circulate, temp controller, and a self-priming pump. Let's just say the holding tank is in the oem spare location.
On the holding tank side of the heat exchanger, add a 12v water pump that's controlled by an automated 12v temperature controller with an ON/OFF switch in the cabin to circulate the holding tank water to temp. While you're driving to camp, hit the switch, the pump circulates water from the holding tank through the heat exchanger to get it to temp, temp is set to turn off the pump when it reaches a preset temp, and have a separate 12v self-priming pump connected to the holding tank for your shower head.
Just throwing some random ideas out there.