For vehicular use (with no onboard water heater) I use a Hozelock PortaShower.
Basically, it's a stand alone container which is filled with hot water and pressurized with the built in air pump.
The shower head has a trigger switch, and the unit can be used to store/carry additional fresh water till needed in it's shower role.
Good design, quality materials and construction make it a well made and long lasting bit of kit which is popular with small motorhome owners in the UK.
I like the utter simplicity of the design, which can be used to clean boots or anything else that needs a wash, inside or outside.
