I found your post a bit confusing -- are you having issues with mice in the basement compartments? What does that have to do with drainage (are you saying the drain opening is where they are coming in -- around the black water hose, for example...)?
I use a stainless steel scrubber (chore girl is one brand...) and stretch it out to use as flexible gasket to stuff in around my flex waste drain when connected up to a park sewer drain.
Quite a while ago I went through my coach and mouse-proofed all the holes I could find. Rigid foam insulation from a window caulking/weatherproofing aerosol does not expand and break things... Then I used expanded metal mesh in all the heat registers, fresh air entry to the dash AC, etc. There is a thread on this board that I will try to link to that has some pictures.
Found it -- in the thread here mixed in with lots of other little fixes...
http://www.gsowners.com/viewtopic.php?f=1&t=2249
Chuck