How about your return lines, are there valves in those lines to divert to a different tank. You should have a manifold setup to where you can draw or return to whatever tank you want. I suggest putting in this manifold so the options are available and this will tell you if one line is plugged in the future. I had the same situation when I bought my boat and brought it up from florida. Turns out the valves at the manifold were set up to draw from one tank and return to the other tank. We started to list and I caught just before fuel was going to start dumping out the vent line. Good Luck!
Tracy & Susie Hellman 1985 Ta Chiao CT 35
Twin Lehman Super 90's Cummins Onan 5kw
Lightkeepers Marina -Little River, SC