Bill has good answers; I'd like to elaborate if I may...
Originally Posted by bernie
If my onboard water tank runs dry will the hot water tank also be dry.
When filling up the water tank how do I know when the hot water tank is full?
Will the pilot light
stay lit during travel or will have to turn it off and relight at destination?
No, the water tank will not drain the hot water heater by itself.
Like Bill says, when you open any hot water faucet and get water
not air, the hot water heater's tank is full.
If you close the Propane
Bottle's valve to travel, the water heater's pilot light will go out (mine takes up to 10 minutes to burn off the propane
left in the line between the Propane
Bottle and the Water Heater.) However, if you forget to relight the water heater's pilot light the next time you open the Propane Bottle's valve, then the propane will flow out the water heater's still open gas valve with potentially dangerous results.
I highly recommend consciously turning the water heater off during travel and relighting it at your destination.