Looks like they are in for a sharp learning curve with RV showers. I've been there as we did most of our RVing with my DW and 3 DDs onboard. If we happened to be RVing where showers were provided they opted to use those in most cases and my son and I would hang back and use the rig's shower.
Since the kids are grown and too busy with college and work to accompany us on most trips it is not much of an issue. My DW showers first and I'll wait 10-15 minutes after before I get in. She never complains about running out of hot water and she has very long hair to wash and rinse.
We use the Oxygenics showerhead that uses less water while producing a more effective spraying action. Atwood also has a 10 gallon water heater which would at least be more productive for your situation. Regardless, showering in an RV means get in and get the job done.