Are you SURE you're loosing water from the vent? There are FOUR low point drains in the Seneca... Three of them are READILY ACCESSIBLE in the Utility Basement. There is a FOURTH low point valve on the OPPOSITE SIDE OF THE UNIT. On my 37TS, the fourth valve is located in the basement rear of the LP. The valve is "tucked up" behind a false wall in this basement. ODD placement, but I know it's there so I can monitor it. Check to see if this valve isn't COMPLETELY closed... That MAY be all it is.
Highlands Ranch, CO
2015 Seneca 37TS ("Screaming Eagle")
2009 Hummer H3 ("Ranger")
2005 Chevy 2500HD ("Max")
2008 MasterCraft X-15 ("One More Time")