It could be the outside top right marker light. Its recessed formed fiberglass pocket. When the coach sits level that shelf can hold water. Eventually the pocket fills and because the wires pass thru a non sealed hole the water seeps in. On my 2014 I found water under the sliding passenger table. And discovered the issue.
Get up there, remove the lens and seal the wire pass thru hole with silicone. Or. Always park down hill!
The other side the unsealed hole was drilled higher up so it does not happen. Same with the three center lights.
Unsealed pass thru wire lights are found under the tail lights too. When ever I have to remove a lens or fixture I find this.
Good luck.