Water Collecting between Black Tank and Corrugated Liner

[RoadKill Bill]

Twice now water has collected between the black tank and the corrugated liner below the tank. I'm not sure how it gets in unless it runs in there somehow during a heavy rain. There does not appear to be a lead in the black tank (had it checked out last time it happened).

When we go to hitch and extend the landing gear the collected water pours out. Since it collects there over a period of time, it also begins to stink. Has anyone else had this issue and what did you do to correct it?

Thanks in advance!