Washing of rigs or tow/ toweds is about camp water supply, and the time of day.
Most camps are on wells which means peak demand is in the morning for washing dishes, showers, etc.
Nearly every camp we've been to allows washing at certain times of day when the wells are not working hard to meet domestic demand and if there is enough.
Ask the camp for permission, during what time of day, be prepared to pay for the privelage.
Water softness is my concern. It does no good to wash the grime off only to have rig or truck covered in hard water spots.