This would work:
https://www.summitracing.com/parts/...MIxu2D0Oe72AIVDnh-Ch2AqgjLEAQYASABEgJG9PD_BwE
When the valve is closed (not energized), the fuel goes to the engine from the main tank.
Open the first valve and fill up the auxiliary reserve tank via the 12V fuel pump.
Close the first valve and start the engine and run off the main tank.
Open the second valve and you are running off the auxiliary reserve tank.
Close the second valve and you are running off the main tank.
If you get 20 MPG, you have plenty of fuel in the auxiliary reserve tank to drive somewhere and fill up the main tank if you ever run the main tank out of fuel.
Running out of fuel is a pain in the ass.
Naturally when you are not running off the reserve tank, you have lots of fuel for the diesel air heater.
You could actually run the diesel air heater and engine off the auxiliary reserve tank at the same time if you wanted.
The only thing I didn't mention was you could T lines to filter the fuel before it gets to the auxiliary reserve tank, but you can control this by where you install the 3 way valves.
This is the set-up for the 110 Diesel Dormobile.