Why is the reserve not really a reserve?
Why does gas disappear from one tank and magically appear in the other?
Why does the generator run out of gas when there is still gas in the tank?

The tank in the rear is the main tank.  The fill tube connects both thanks at about the 1/4 full level.  Both tanks fill to the same level once past the 1/4 level.  The Generator draws from about the 1/4 level so that it will not draw the tank dry.  The Engine draws from which ever tank is selected by the selector switch.  This switch also selects the correct gas gauge for the tank selected.

If the main tank (rear) is selected and used until it is empty, there remains about 7 to 9 gallons in the Auxiliary tank, unless you go up or down hill as you will soon see.   The generator will not run when the main tank is below the 1/4 full level.
 
 

When you go up hill, the gas runs from the auxiliary tank to the main tank through the fill tube..  When you get level again, the gas will seek the same level in both tanks through the fill tube.  If you had the Aux tank selected before you went up  hill, a lot of your gas gauge reading just went into the main tank.  Because of the position of the fill point, it is better to drive in the auxiliary position as most of the gas will end up here in up and down hill, driving.  Some owners have reversed the connection to the selection valve so that the front tank is the main tank.

If you go down hill or park down hill, the gas will flow to the Aux tank.  The generator will get no gas if the tanks are low.
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Result #1:  Because the fillers enter the tanks so far forward, there is very different transfer behavior between the 2 tanks depending on whether you are going up hill or down hill.
Using the numbers below,

On level ground, one tank can drain the other down to 40% of full.
On a 6% uphill grade (pretty steep),
        The rear (main) tank would leave over 60% of the fuel in the
front (aux) tank, and miss some of its own).
        The front tank could get next to nothing out of the back tank.
On a 6% downhill grade,
        The front tank could drain all but 19% of the rear tank.
        The rear tank could pull about 20% from the front tank, but
could not pick up some of its own fuel.
 

Here are the numbers I used.  Got them by dragging myself around awkwardly under the coach.  If you have more accurate numbers, let me know.

    Height    5"
    Length    27"
    Width     48"
    Bottom of filler above bottom of tank    2"
    Center of filler from front of tank          3.5"
    Tank center-to-center distance              32.5"

Dick Elsley
 
 

For those who have put on a TBI and have the return line going into  the lower end of the filler tube there are some other effects.  I  have found that when my tanks get down to the 1/4 full level I have  to keep switching my tank selector back and forth as I drive because  the fuel pump is pumping gas back to the tanks and it is flowing  through the fill tube to both tanks even though it is pumping out of  only one.  Emery Stora