Hmmmm. If what you found in your Argosy was an AC compressor type fridge, it must have been an after-market modification. I am not aware of any camper put out by AS that didn't have an ammonia evaporation fridge, except for the very early vintage trailers that only came with ice-boxes.
Dometic and Norcold are the most common vendors. I believe Dometic is the one AS uses most.
Now, the tricky thing is to look for an LP line running to the spot in the trailer where your fridge is. Hopefully, it's still there. The way these lines are usually run is from the LP tanks, underneath the belly pan (never hidden inside, because of the risk of a leak, leading to buildup and BOOM), then straight up through the floor to a direct connection with the appliance.
If it's not there, you can add a line. If it is intact, you might want to test it for leaks via the soap bubble method.
Next, carefully measure the original opening for your fridge. Somewhere on the web, you can search for models made by Dometic and Norcold that most closely match your space. Note that they make changes over the years, and you may not find an exact match for 1978.
As an aside, I own a 1969 with the orig. Dometic! When I got the trailer, the fridge had a number of cosmetic issues: there were stains on the inside I could not get out, some of the plastic shelves on the inside of the door were broken, the wire rack had lost some of it's chrome platting and had corrosion, the fridge door wasn't so good, etc, but it still worked!
So I figured I would just wait until it died, then put in a new unit (which runs about $1000, btw, new!) But, 15+ years later, it still works! Some other vintage AS owners have told me that when it dies, I would be much better off to fix it rather than replace it; it would be cheaper and last longer. They said the new ones only last about 5-10 years. And that if I did get rid of it, to let them know, because they would buy it!
So, if you're lucky, you might find a vintage fridge out there somewhere.