Navy showers are short, methodical showers intended to use minimal water. 1-2 gallons of water per shower are possible.
The general process is:
Practice this procedure in your home shower before hitting the road.
These typically rely on the Navy shower method due to the absence of cheap power and water off-grid.
Truck stops have shower available for purchase. These amenities are designed for truck drivers but are available for anyone. Couples can generally shower together for the same price.
Pilot/Flying J have rewards programs where every 14g of fuel accrues credit toward a shower.1) Love's has a similar program but it requires 50g diesel fill-ups, so it will not apply to many 'dwellers.2)
Some folks check into a hotel for a night every once in a while to take a long hot shower, sleep in a “real” bed, use the microwave, etc.
Planet Fitness is a popular option for getting showers while in civilization. Note: during Covid-19 call ahead to make sure the showers are open.