RE fuel tank....these guys (
http://www.aerotanks.com/vehicles) build the tanks for agile. They have an in-wheel well option for the fuel filler - really slick. The photos show the fuel neck in the cargo area. Aero offers a 20 and 30 gallon version...and will weld on an integral skid plate for a nominal charge.
Fuel is pumped from the aux tank to the Sprinter tank via external pump & filter and controlled with a simple mechanical timer. They will fit a gauge/sender to the aux tank and connect the transfer pump to either the main or Aux battery. This simple method completely avoids MB shenaigans. The 12v pump runs while driving or not. It is timer based...10min will transfer 10-15 gallons and they said there is no issue with letting the pump run dry for a short bit. I don't believe there is an auto shut-off if the main tank is full so some caution must be exercised.
If the Sprinter is running, the dash gauge may not update until the key is cycled.
I will be installing one of these soon - but will most likely have to remove the rear heater blower to do so...(I have a 144 passenger).
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