My advice would be replace the whole thing. You can go around looking for an exact fit lens for a nearly 40 year old trailer, or you can order a couple full assemblies on Amazon for under $40 and swap them in with a couple of screws and some butyl tape.
2011 Jayco X19H (purchased 2015)
2008 Jayco 1007 PUP (purchased new, traded for the X19)
2018 Nissan Titan Midnight Ed.