+1 on the diagram.

If all the air is not purged from the water heater, it might act as an accumulator and push some water out. However, it still won't be able to go below the level of the output since then only air would be able to be pushed out. If the pump is running dry and pulling air only, it will push air into the water heater tank but still the output of the HW tank is the lowest level to which it can drain. I don't know where that output is in relation to capacity. Perhaps a quart or two is all the "reserve" that would be available.

The water pump draws directly from, and only from, the holding tank. The only thing the pump can pump dry, is the holding tank. It also cannot push the water out of the WH, because once it draws air from the holding tank, it's done, until the holding tank is refilled and the pump primed.

I guess I'd like to see a diagram of how the water heater is plumbed.  To fill it from the fresh tank, you have to pump it from the tank to the WH.  To get hot water out of the water heater it seems like it has to be pushed out by cold water being pumped in.  I can imagine gravity playing a part here, but I'd think there must be a check valve, or something like one, to keep heated water in the tank when the pump is off, so how can the WH tank be used as a traveling reserve for fresh water? 

It won't amaze me if it works as you say Mike, but I'm having a hard time figuring out how to do it, short of having the water heater up on the third floor.

TT

Stephen is exactly right, but I feel compelled to expand on the anode situation.  There are 3 significant components to your water heater, aside from the tank.  There is the electric heating element.  It has wires going to it.  There is the propane heating apparatus.  I've never gotten into it, so I can't describe it, but it has a propane pipe, a flue, an igniter, and some kind of burner.  The functions of both the electric element and the propane burner are the same: heat the water in the tank.  Then there is the anode. It screws into the tank, with no wires or pipes.  It does as close to nothing as possible.  This is a bit over my head in terms of scientific details, but the anode just slowly dissolves (over months or years).  It dissolves to protect the hot water tank from being dissolved by electrolosis or chemical issues.  It's often called a sacrificial anode because it sacrifices itself to protect the tank.  Your water heater will work without it.  You can replace it with a galvanized pipe plug (that will do absolutely nothing except keep water from flowing out of the hole for the anode.)  It won't burn up no matter how empty the water heater might be.

Turn off the water heater the instant you realize you're out of water.  Electric and propane.  Much better, don't turn it on unless you know the WH tank is full and you are sure you have plenty of water in your fresh tank or you're hooked up to "city water".

TT

The 6 gallons are your water heater. It is not a reserve. You are right to be concerned about it, but the anode rod is the least of your worries. It won't get ruined from being dry. What will happen though is if you have the electric part turned on and the water level drops, the element will almost immediately burn out. It can be replaced, but avoiding the problem is better.

What will happen is that the pump will start to pull air instead of water and you will hear and see those bubbles and spurting when you turn on a faucet. Those bubbles will go through the water heater tank but it takes water coming in for hot water to be pushed out.

If it can be largely emptied, I should think that there is a real possibility of then accidentally ruining the anode. 

(I carry a reserve water container and it hasn't happened to me, but sometimes I run pretty low on water)