Portable showers (for camping and nomadic living) have the advantage of being useable inside the vehicle (if a shower stall is present) or outside the vehicle in a shower tents, behind a tarp attached to your vehicle, or out in the open. There are three main types of portable shower systems (also called "camping showers" or simply "camp showers"): Pressurized water with onboard heating, pressurized water without onboard heating, and gravity-fed water.
Pressurized water WITH onboard heating
Portable propane camp showers are a good option for those without a pressurized water system in their camper. They heat the water to an exact temperature and provide water pressure.
Pressurized water WITHOUT onboard heating
Portable shower pumps require you to heat the water first by other means (such as on your stove or campfire). The hot water is poured into a bucket, checked for correct temperature, and pumped from there. Another option is to use a battery-powered garden sprayer, or a pump-powered one.
Gravity-fed water
The least expensive portable shower option is probably a solar shower bag to heat the water in the sun then hang it inside for the shower. This does require a bit of planning to set the bag out ahead of time, and won't work on cold days. Note that the clear side of the bag (if present) should face toward the sun, and the black side should face away from the sun. Get a solar shower with a durable hanging hook, as those are prone to breaking. On colder days, you can set the shower bag on your vehicle dashboard to warm it in the sun, but only if you are very sure it won't leak.