Fact #1: you can use the electric element and the propane burner at the same time.

From this link: http://www.rvcomfort.com/suburban/products/water/faster.php

"Fast recovery means more hot water and fewer cool water cycles. Six-, 10-, 12- and 16-gallon Suburban gas water heaters feature 12,000 BTUH input and a recovery rate of 10.2 gallons per hour - that's the fastest in the industry. Combination gas/electric models also use a 1,440-watt element to recover an additional 6.0 gallons per hour at campsites. For higher recovery, both the gas and electric element may be used simultaneously."

Fact #2: if you drain your water heater and forget to turn the electric element off, the next time you plug into AC power the element will burn up faster than you can say "oh, I forgot to turn off...".  Poof.  So remember to turn it off after every trip.  The switch is in the lower left side on the outside of the water heater.

Fact #3: Filling the fresh water tank does not automatically fill the hot water tank.  After you start filling the fresh water tank, turn on the water pump and open up a hot water faucet.  It will spit air and water for some time while the hot water tank fills (with about 6 gallons of water).  Once water flows normally you can turn off the faucet and turn off the pump.  Don't use the overpressure valve on the hot water tank to let the air out of the tank since you'll also remove most of the air cushion that's supposed to exist in the top few inches of the tank for expansion.  Once the fresh water tank is full, you are done.  If you will be boondocking its essential to do this before parking in the woods since you'll end up sucking up 6 gallons of water from your fresh water tank when you can least afford it.  If you'll be using city water you can fill the hot water tank with it, too, just be sure you do it before turning on the electric element.

Fact #4: Once you run out of fresh water, you won't drain the hot water tank.  First, the water pump has a backflow valve, so water won't fall down into the fresh water tank from the hot water tank. Second, the water pump pumps water very well, but not air, so once the fresh water is gone there is nothing that can push any of the hot water out so it just sits there.