If the heater's on, maintain pressure..
I think question reversed.. I f you're going to use the water heater, then you should either be connected to "city water" with a hose and valve opened, or the pump should be on to maintain pressure from the tank. If you're not confident in pump, and leaving the trailer, turn off the water heater when you're away...
In Theory, there's no difference between Theory and Practice, but in Practice, there is usually a difference...