there is a thick black foam rubber gasket that should be there, attached to the bottom of the drain down pipe on the sink. Usually I can find just about anything on Amazon but so far came up empty on this.
I will say that it reminds me a little bit of the foam gasket that fits between the toilet tank and base on most toilets; you might be able to adapt one, but I think the toilet tank hole would be much bigger so would probably require some cutting down to fit it.
Hopefully someone else can chime in with a parts source for you.
2011 Jayco X19H (purchased 2015)
2008 Jayco 1007 PUP (purchased new, traded for the X19)
2012 Chev Traverse LT with TP