Water heaters come in basically 3 RV sizes
Six gallon (enough for one person to take a shower if they do not dwadle or use a cut off valve, more on those in a second)
10 gallons, (Also 10 gallon in a six gallon hat, But that need not be explained her) Two people might get lucky with a cut off valve or one longer shower.
Demand (Tankless) all the how water your propane tank can heat. Nice LONG hot showers. However at low flow they sometimes fail. These units only eat Propane (The others can eat electric or Propane).
The cut off valve:
Cut off valves come in two flavors. TOTAL cutoff, (I used to use one) and what Oxygenics calls "Smart Pulse" (lets water trickle/drip when turned off (What I use now).
The problem with the full cut off is often when flow is restored there is eitehr a HOT or COLD "Burst" before normal temps return.. The trickle seems to prevent that.
The 10 Gallon in a six gallon hat (This is an Atwood product) heats the water much hotter than noraml (normal is 120 degrees) and then uses a thermostatic valve to mix cold and hot together as it leaves the water heater so you get 10 gallons of hot (120 degree) water out of six gallons of Very hot (160 or 180, not sure) Degree water. Nice... Unless the mixing valve fails.