I do not wash my hair every day. I use the shower every other day if boondocking to do that, Or wash it in the sink as well. When I smoked, I couldn't get away with that, but since I am not dragging my head mop thru the mud all the time, it is fine skipping a day.
Water conservation is the goal, and you can get squeeky clean with a sink of hot water and a good terri cloth washcloth. It's not as elegant and comfy as the shower, but it does the job without too much damage to the water supply.
Showers are very wasteful when you think about it. They are NICE, for sure, but when camping, I give up the warm fuzzies of one and still get clean.
Believe me, the sink in the bath is far more elegant than doing that in my 13 with the kitchen sink. At least I have a water heater in my Big Burro
, so it's easy.