My in-laws gave us a Zodi portable propane water heater/shower unit. This thing screws onto a 1 lb. disposable propane bottle [optional adaptor for tanks available] and uses 4 d-cells to power a pump into a small shower head.
I admit that at first glance this thing looks like some of the dozens of POS camping gear I've had over the decades.
Now that we have used it to take several showers I'm more impressed. The batteries seem to last a long time and we got 9 or 10 showers and still have some fuel left in a tank that has already powered our propane space heater for a couple of hours.
If your supply water is VERY cold you can pre-heat it with the system in a couple of minutes and it only takes about 3-4 gallons to get a shower - although if you have really long, thick hair, the shampoo takes a while to rinse off - it's not a real huge flow rate obviously. It's a continuous flow heater which means that you must not let the pump stop or the water run dry - the unit would almost instantly make scalding steam. We found it useful to shower as a team - one tending the heater, the other getting wet - this makes sure you have plenty of time.
The YL made a tent herself to use as a privacy screen and we bought a teak duckboard about 18" square from Ikea for $5 for standing on.
They have a double-burner model as well. If you were running an outfit, this might really be useful for your shower tent.