Ummm, I see no one answered the OP, but reading this today my reaction would be, don't put screen over the duct entrance, get the mice permanently out of your TT! trying to trap them inside your duct work is a whole new problem!
Lots of good posts here on things to do to mouse proof your trailer.
2011 Jayco X19H (purchased 2015)
2008 Jayco 1007 PUP (purchased new, traded for the X19)
2012 Chev Traverse LT with TP