Okay, I have done a little investigation now.
Bad News,... there does not appear to be any way to isolate (valve off) the shower from the balance of the system. There just isn't enough room between supply tees without also cutting the flow to the sink HC supply. It is also down hill from that intersection.
Good news,.. It appears to me?? If you, 1) shut down the water pump; 2) shut off the hot water heater; 3) open hot and cold sink faucet; 4) attach outside shower nozzle, open HC valve and let hang down; 5) open the in line valve that you open when filling the tank,.... Then all of the water in the system drains back into the tank (IF YOUR TANK IS EMPTY ENOUGH TO RECEIVE IT!). Then, 6) start and stop the water pump a couple of short burst to free any water from the impeller. All valves stay open while traveling...
Granted this is not a perfect winterization, but along with warming the interior during transit, it should remove solid pockets of water that would expand and break anything. You also don't want the pump and in particular the impeller area to have frozen chunks of ice in it at restart either, thus the need for heat under way.
This is easy enough to do at the start of a day of travel and then once you get to camp just start pumping, bleeding air and heating water. This leaves the tank drain valve..... I am going to look at hollowing out a rigid foam cover to Velcro over that, and maybe with room for a chemical toe warmer. Chemical warmers could also be placed along with some insulation inside the external shower door. These two simple procedures may negate the need for draining, but I'm a belt and suspenders kind of guy. With a little more experience, I might change that.
I'm inviting other opinions and reinforcement if you think I have it right.? (Hopefully based on experience).