I would suggest checking the vent/overflow side of the tank. Not sure how its vented but it should be, air has to have some place to go when filling, and prevent overfilling of tank. If vent is plugged or blocked it will restrict its ability to drain. Some manufactures even installed valves on the vents to prevent tanks from siphoning water out of them.
08 Ventana 3330
"Those who stand for nothing fall for anything"
2008 Newmar VTDP - 3330 - ISBXT 350 Spartan NVS
Toad - 2013 Mini Cooper S or 93 Nissan 4WD PU
FMCA F113720, NKK 18573