It seems like you're sucking air from the tank siting between the engines. It's most probably a top mounted pickup tube. Sometimes they rust and develop pin holes along the pickup tube. As long as the level is over the hole, it works fine. When it gets lower than the hole, you start picking up air. I suspected that in my water tank and went in with a snake camera. Turned out to be something else in my case but it's frequent if the tube is made of steel. Other possibility is that your pump is just not strong enough to pull that far from the tank. When it's full, it's easier for the pump to pull water. That was my case. Hope it helps. You should have a valve to switch from one tank to the other. Otherwise, when one is empty, you will start sucking air...
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