From your description, it sounds like you are not sucking gasoline, but the vapors above the fuel level. Are you sure that the refueling intake is below the level of the gasoline? There might also be a crack in the tubing, as it would be easier to pull air from a crack above the fuel level than the fuel itself. Though you probably would have a powerful enough pump to pull both 1 GPM fuel - even if there is some vapors coming out with it.
I am also worried that there would be something still stuck, such as another check valve or device made to keep the fuel from leaking out while the pump is turned off.
You might want to contact
Transferflow.com because they used to build fuel tanks for several toy hauler manufactures. You might get lucky and they know a solution to your problem. But they will be closed until Monday.
Fred.