Just so others are aware, the quechua portable shower, which we have, squashes sort of flat, so if space is a problem for you in the car you might want to choose that over the other one autopilot recommended.
When I was visiting my ex-inlaws in Jordan in wintertime (when their sun-powered water heating wasn't sufficient), they used to heat water in a kettle and mix it with cold water in a bowl. You don't need much boiling water to make a big bowl of 40 degrees celsius shower water! Take a big cup to scoop the water over you, and off to the bathtub...
I would use this method in a shower tent, using a little inflatable padling pool as the "bathtub".
The method is also wonderful if you want to avoid paying for hot water at a campsite shower.. I've been seen carrying a dishwashing bowl with hot water from the tap at the sink to the shower cubicles...
------------- Proud owner of a 1987 Sprite Alpine 370 EK, a cheap popup tent and a beloved retro Trio frame tent from the early seventies, called Giraffen.
I can vouch for the Hozelock Portashower (eBay Argos store £15), Quechua pop up shower tent (Decathlon £30) and duck board (bargain shop £5.99). I used this setup for the first this year instead of queueing up for showers and it worked much better than I anticipated. It was good enough for two of us to have a warm shower in the morning. (This was camping at Spa, Belgium for the F1 grand prix and staying on a field where facilities where limited). I boiled up 2 ltrs of water and added this to 2 ltrs of cold, just about the right mix.
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets