There's positively something wrong with the vent pipes between the tanks and the roof. These are atmospheric vents that are designed to allow the tanks to breath. While dumping, they're plenty big enough to allow all the air necessary into your tanks to drain properly. I'd bet when you're able to try the roof top test of dumping water down those pipes, they're going to fill up and confirm your problem. Hopefully for you they go straight down into the tanks making it easier to run something down through them to clear the blockage. It might be those pesky muddubbers built their nests in the pipes and blocked them. That would be an easy fix because they'd be right near the roof and they're water soluble. Good luck in your quest!
