I wonder if the tanks have been plumbed together wrong? The aux tank in mine drains until it's empty (that's 70L of Long Ranger) then I start to see the fuel gauge move for the fuel in the car's original tank (80L). In mine, they chose to set it up so that I couldn't see what the fuel state was until the aux had been drained. I'm used to that now.
I was told that I could have it so that they both drained together, but that might cause issues with running out of fuel (pickup sucking air) even with a few litres left in the tank.
I imagine that it would be possible to set it up so that the main tank drained first. Or that the main tank wasn't even in the loop. It's probably best to track the plumbing!
There should be two fuel hoses going to the engine bay - one is a fuel pickup, and one is a fuel return. There should be other hoses between the main and aux tanks. Make sure that the filler goes to the aux tank, and there are hoses going from there to the main tank, and the two hoses going to the engine bay are connected to the main tank.
If it's not set up like that, you could ask an ARB store to take a look at it and give you some advice.