It is the Anti Siphon valve causing an issue. The tanks are T connected and the difference leaving you 3 gallons shy is one dvalve having more pressure than the other. They don’t all do this, though some do. We did not use Anti Siphon pickups years ago and a qualified wrench could remove the check ball on older one, with the Fuel Demand Valve we have not checked on it. Make sure the right and left hoses are the same length also, as if one is longer that also could be a portion of the issue.
Another thought is that one pickup tube could be a little short. Removing them and measuring the length would tell that story. We do not install the pick up tubes as they come pre-installed.
The last option would be place a T valve in line and switch those tanks also.
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