speaking as a veteran hairdresser,,, I don't think there's anything in the haircolor chemicals that's bad for your sewer in your camper. The only thing that's in haircolor is ammonia and peroxide and those are used in cleaning agents anyway. If your scalp isnt harmed... your holding tank sure won't be harmed,, and besides,,,you dump it at LEAST once a week,,, i'd think. But rest easy on the tank part..
As for rinsing,,, i'd get in the shower,, just like I do @ home, when I do my own hair. I'd be athinkin' the bigger problem would be the application of the color and if you had a big enough mirror to be able to use a hand mirror and see the back of your hair... ??
If you can shampoo your hair while you're on the road,, you can COLOR it too! Same difference,, w/ the rinsing,, I mean... right???
REPORT on your progress... i'd like to know how you made out.
JA