Economical water heater for shower water:
If you had barbecue on the campside and no warm water here is an opinion:
-1m Silicone hose D10mm
-2m copper pipe D10mm -
the printing part for the decathlon shower or
-the printing part for the Hühnersdorff canister
-a camping grill or another fireplace
Twist the copper pipe around a bottle, so you get a coil like in the picture. Connect the silicone hose by putting it on the copper pipe and on the printed nozzle (or stick it in the connection of the decathlon shower) and start the fire.
The silicone hose on the outlet of the copper pipe (coil) has to be as short as possible, so you need no one way valve.