Many RV Lavatorys are custom made for that layout and are often smaller than home types.
All you can do is take some measurements including the depth and go to your local big box home center and start comparing sink sizes.. You might be able to find some smaller sinks that may fit with a little enlarging of the opening..
Alternately you might want to check with some mobile home dealers. Mobile homes typically use smaller sinks than what you find in a sticks and bricks home..
Baring all that if nothing else there are surplus RV parts places which buy up new old stock and resell it..
Here is a couple of places I have purchased RV parts from..
RV PARTS NATIONRV PARTS OUTLET