We fulltime, so this is from a fulltimer who stays in campgrounds on full hookups.
With tankless water heaters, the water faucet or shower is USUALLY either turned on full blast or it's off. There are some exceptions to this statement I'm sure.
RV LP water heaters (tank) are pricey for what they are.
AC/LP RV water heaters are more flexible but are pricey. We had the AC/LP type in the Class C. When the old 6 gallon Bowen AC/LP water heater developed a leak, we replaced with another similar water heater
with no pilot light (Atwood.... the company who acquired Bowen). From 2007 - 2010 we never used the LP side, just the 1400watt electric heating element.
AC only water heaters. We put a 10 gallon electric water heater
in the bus. I wanted a 20 gallon but didn't have the room. Takes about 20 minutes to heat the water up. I can take a really hot shower that lasts a little over 10 minutes. Downside... this water heater is sucking up almost 2KW per hour. So we have been unplugging until we need hot water. This explains the massive jump our 30 amp electric did in the month we installed the heater (our site includes $60 electric allowance... I have to add on an additional $100 to our next rent bill for electric bill overage). We need to pick up a switch and wire it into the water heater line. Kinda silly and wasteful to heat water while we sleep anyway. I can turn the water heater on for 20 minutes, take a 10 minute shower in the early evening (before 7PM), turn off the water heater and I still have warm water the next morning when I get up (3AM). The cold water is numbingly cold. This is a well insulated water heater and it sits under the kitchen sink... directly on the floor to clear the sink drains.
I bought a Killawatt EZ
but can't use it on the water heater (2000watt heating element pulls too much electric). But I can use it on the electric space heaters. They are not the problem. It's the water heater. Maybe at some point we might replace the heating element with a lower wattage one. By that time, we will most likely be accustomed to switching the water heater off and will continue doing that even if we do change out the element.