There are several ways to stay clean in the van. None will be as quick or easy as hopping in the shower in a home or apartment.
In-vehicle showers require space and water, both of which are in short supply for vandwellers. Class C and larger vehicles have the space and water tank capacity to make regular showers a reality.
Regardless of the type of shower, water conservation is key. With practice, a navy shower can take ⇐ 1 gallon of water. Shower bags have valves to facilitate navy showers.
The easiest way to get long, hot showers is to have a gym membership. The most common amoung 'dwellers is the Planet Fitness “black card” membership, since it allows use of any PF nationwide.
A sink bath is very simple. Put some water in a sink or large-ish container2) and use it to wet and rise a washrag, scrubber, or sponge. Wash a small area, wet and wring the rag, repeat. Soap is not required for this process. If you do you soap, consider having a seperate bowl for rinse water.
When using single containers for baths, consider using the army procedure:
The washwater can be used to rinse pee bottles, etc.
Baby wipes are relatively inexpensive and available everywhere. They are particularly useful for quick “pits and bits” touch-ups.
Wipes are also useful for folks in very small spaces((like car and minivan dwellers), since it doesn't require large, open containers of water.