That panel is not unique to Airstreams. I'd stop by my local mid- to large-size RV dealer with a shop and see if they have any old ones laying around. I think all you need to do is to provide the dimensions and say it's for a '79 Dometic. (Maybe you shouldn't mention "Airstream" right off the bat.) The panel was missing on my Argosy when I got it, and Three-Way Trailer in Atlanta sold me a used, but nice, one for $5. I painted it to match the trailer, and it's been on ever since.
1975 Argosy 28 "Argosy"
1979 Excella 500 31 "Betsy"
1992 Lincoln Mk 7 LSC
2003 Dodge 2500 Cummins "TowHog"
"Lucy Loosehair" the cat - Airstream mascot
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