Yeah - sounds like a little more understanding could improve your survey. Here's a summary of the above, with a some added ideas.
- Many RVers camp with full hook-ups, so water supply and waste tank supply is unlimited.
- Even if the hot water heater capacity isn't sufficient (which is rare), there are on-demand water heaters where tank capacity isn't a problem.
- When dry camping, freshwater supply is a limiting factor. In this case, creating a higher-pressure and lower-volume shower head would really improve shower performance for a given level of water use. Check out Oxygenics brand shower heads.
- Also when dry camping, many people just use water to wet themselves down, then they turn off the water to soap up, and turn it back on to rinse. That's a Navy shower, military shower, sea shower, etc...but I think the intent of your study is to have a more comfortable traditional shower with no behavior modification. Still, something to be aware of...some RVers don't care about this, and some would prefer a traditional shower all the time, and that could skew your results.
Hopefully something good will come from your study (better than "just an A")!