The patch might have covered a window above an original ice-electric Marvel fridge, similar to the window on the earlier FC pictured here:
and the fridge seen here:
Is there any evidence inside the trailer that a short Marvel ice-electric fridge used to be there?
I have not seen many drip caps above added-on flat vents for fridges, so think that hole originally was a window. Then again, maybe a former owner at first replaced the old window with a rectangular louvered fridge vent but didn't like how it worked, so had the Calif. factory add a long oval roof vent for a propane fridge. Then perhaps he patched over the old window/rectangular vent as the new roof vent made the old vent obsolete.
The long oval vent on the roof above the patch is a California style fridge vent. It's presence there suggests a propane fridge was installed at some time in your trailer.