I go to the convenience store that sells coffee and get boiling water out of the tap where they make coffee into my thermos.
I drive to a private spot. After dark, that spot can be mostly anywhere.
I add a little cold water to the thermos, if the water is still too hot after the drive, cold weather, etc.
I have a larger container on the passenger seat that the thermos can sit in and not tip over with the lid opened or closed.
I start by getting my hair untangled with a small amount of conditioner on the ends.
I put a wash cloth into the thermos, wring it out, and go over my hair downward as I sit in the drivers seat.
I then shampoo my hair.
I use the same wash cloth to wet and keep going over my hair until the soap is out, wringing it out in between.
I condition my hair a little on the ends, if it got too clean and will get tangled.
Note: I never turn my head upside down anymore to wash my long hair. It gets my hair too tangled and I loose too much hair this way.