We usually turn on the water heater when we're a few minutes from stopping for the night. It does have the feature of using the engine's hot water to also pre-heat the water in the water heater.
It could certainly be a bad pressure gauge but when I attach the hose for city water or turn on the pump it shows 45 pounds each time. I can turn a valve to add water to the tank and it snaps back to 45 pounds when static pressure returns. I often cycle the water from the pump through the tank to get all the air out of the lines and once again the pressure gauge always reads 45 pounds.