The absolute hands-down best shower head is an Oxygenics BodySpa
. It will take your weak and weinie water pressure and turn it into a satisfying shower experience (there now, do I sound like an advertisement?).
Most rv water heaters are 6 or 10 gallon. If you like really, really long showers, a 10 gallon will produce all the hot you need. We find that our 6 gallon is adequate for standard showering for the two of us and more than ample if we take Navy showers.
Other people have installed the tankless systems, but I've heard mixed reviews on them in RV applications.
Do you intend to travel in your RV at all? It sounds more like this is a stationary tiny-house kind of plan. Because what is feasible in a stationary application that never moves and is hooked up to sewer and city water pressure is going to be difficult when you are limited by the size of your fresh-water and gray tanks, as well as your water pump pressure.