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.