The shower is not the weak link in my RV. Mine has a standard 6 gallon Atwood water heater. The initial warmup is about 30 minutes, and if the switch is left on the water heater thermostat will keep water appropriately hot. Shower time is more than sufficient when hooked up to utilities.
Length of showers: Plenty if the tank capacity is sufficient. If dry camping shower length is limited by tank capacity. I cannot think of any method other than recirculating shower water which could extend the time. Recirculating water would be the only potential advance in RV showering, and there are lot of pitfalls there, debris, odors, bacteria, etc.
In my opinion the one RV system which is long overdue for an upgrade is the generator. These need to be fuel injected to reduce or eliminate the "maintenance run time." In many cases generators accumulate more hours for maintenance running than for actual useful work. Fuel injection would also allow for automatic altitude compensation and much cleaner exhaust via a catalyst. Any engine running in close proximity to people needs to be as clean as possible. The technology is there, has been for years.