Jo,We used to use a black thermal bag type thing that was hung on the side of the tent and heated up from the sun. It had a shower attachment attached. It was sufficient to wash a couple of kids. I have seen them still at dealers and towsure.
The temperature did depend of course on the time of year and the weather, but we found it most useful when rallying in the summer months at farm type sites. The kids were hot and dirty at the end of the day and it freshened them up. There was also no problem with the waste as we just moved the tent a bit.