I had a 2009 Grand Banks Europa for a number of years with a similar setup, and struggled with the same issues. However the details may be different, so don't apply this literally.
I wanted to operate with the two tanks connected and acting as one, simply for the convenience of having a larger working supply of water, and not having to play around with tank monitoring and switching from one to the other.
My memory may be a bit fuzzy on this, but here's what I recall, in no particular order:
- The tanks were at different elevations, both their tops and bottoms. In my case the forward tank was lower than the aft tank.
- The plumbing allowed the pump to draw off either tank, or to have both tanks and draw off both. The later is how I operated.
- I would fill the aft tank, with the expectation that water would flow to the forward tank, filling it first because it was lower. Then when the pump would draw off the water, I expected the higher aft tank to empty first, then the lower forward tank. But this didn't work as expected.
- The problem turned out to be a low spot in the forward tank's vent line. The low spot would flood, and then the standing water would block the vent. The blocked vent would prevent water from flowing from the aft tank to the forward tank, both when filling the tanks, and when drawing water out.
- To work around this, I would crack open the deck fill for the forward tank when filling. You could clearly hear the air release as the tank vented, and water then flowed freely. Then over the course of using the water, I would periodically open the deck fill again to allow the water to equalize.
- The solution would have been to reroute the forward tank vent to ensure there were no low spots, but I sold the boat before I got around to doing it. The vent line of course ran through a difficult to access area, so I kept putting the project off.
Hope this helps. It was all a good lesson in the importance of ensuring that hoses are self-draining to prevent air locks.