This is a real opportunity to do that fuel
polishing circuit. Using the day tank as a header with filtration and water removal
going to or from one an overflow return circuit. Any doubts about the engine fuel
and you can just recirc it.
If you do access the main tank make the engine
take-off a little higher (out of the sediment) than the recirc take-off.
Put a coarse filter in the line to the day/heater tank and engine quality filters in the return to the main tank. Might be worth fitting a by-pass so you can use the lost
drops of fuel for the engine, even the heater tank contents might be useful on a bad day ! ! !.
I've got the unreliable make of heater, great when it's running, but - .