Welcome to the forums. This is a great site and lots of helpful folks will step right up! I would post a pic of the lock you want to replace if you have the old one... or at least ask someone that has a 85 Excella to post a picture of theirs. Everyone here likes pictures! You might do a search for 85 Excella or door lock, etc. and see if there are threads that pop up. Also look above at the classifieds and search there as well.
There are also several vintage trailer supply places that might be able to accommodate you... it is always a shot ...
I would try: http://www.vintagetrailersupply.com/
they have been helpful to me.
Good luck, and again, welcome! Any questions, just post. You probably won't have to wait long for an answer...
Mrs. NorCal Bambi (traveling in S Tardis)