Doing a little research, I found, on the Lippert site, an O-ring, for a 2004 model, that may be the same as for your rig. If it's the right size, you probably can source it locally from an auto parts store or hardware store. If not, Lippert can probably sell you one.
O-Ring; 3 47/64'' Id; 4'' Od; .138 Dia
https://store.lci1.com/rpb-o-ring-3-...d-4-od-138-dia