I have an LS8-18. I usually fill only the inner tanks since when full I am essentially at max gross. The outer tanks will take on water and I know how it happens in my ship. The vent tubes for both the outer and inner tanks exit into a small chamber in the wing root. This chamber has a common drain on the bottom of the wing. You will identify this chamber in the wing root because there are two plastic plugs that fill inspection holes into the common drain chamber. The plastic plugs allow direct access to the drain tubes from both the inner and outer tanks. When I fill the inner tank until water drains out the vent what is happening is that the overflow from the inner tank is filling the common drain chamber in the wing root and water can back flow into the outer tank.

I am confident that this is the problem because if I am careful to slowly fill the inner tank and unplug the fill system at the first sign of water draining, there is no water in the outer tank. Also, when there is water in the outer tank after filling the inner tank I will drain the outer tank and it stays dry. If there was a leak in some kind of tube inside of the wind the outer tank would continue to take on water after I drained it.