Not sure yours is the same, but most newr Rvs' have a cross over valve that you can turn to direct which direction or tank you want to use. you do need to open the tank that the valve is turned to.
You can, as long as you have water in the heat water by propane heat:
To do this the gas lines will most likely need to be bled of air. You can do this by:
lighting all burners on kitchen stove.
To heat by gas, with propane on, water in hot water heater, turn on interior switch. It may take several tries until all air is purged. You should be able to hear it heating from outside if working.
Flagstaff 8528 ckss fiver
2008 Ford f350 dually 4x4 desiel