Most fuel pickups, returns, and vent tubes are in the top of the tank. You could have algae or other debris blocking the pickup. Does the cleaned filter/separator stay clean? A failed fuel pump could be the culprit. But I’d check the tank vent hose which may be plugged or have a stuck valve. On our coach it comes out of the tank top and runs forward.
A good move would be to call Dave again, as he is far more knowledgeable than me, especially regarding the 3126 setup, and can proffer other suspects.
You need to update your profile or signature with coach model and year, as we won’t necessarily remember it.
Joel