I know on my 2014 Bounder 35K the propane is for fridge, water heater and coach heat when you have no electricity. My Fridge will automatically swap to propane when electric is not available. My water heater has a switch for on/off, gas or electric .
As for traveling with the 3 tanks I believe it is recommended to travel with at least a few gallons in your black tank with a black tank decomposer tab in it. I usually travel with the Grey tank empty if possible. My fresh water tank holds 80 gallons so depending on travel distance I usually put it at half the tank .Fresh water is for drinking water, sink use and toilet use while on the road. At about 8lbs a gallon for water a full tank adds a lot of weight.
Don