The "50 amp service" doesn't matter.
All the converter does is provide stable
12v DC to your trailer for your
12v lights and such.
The converter plugs into a regular 120v AC outlet in your trailer then converts this into DC current. The converter doesn't care if your trailer is 20, 30, or 50 amps AC as far as shore power goes.
Get a Progressive Dynamics Intellipower as I've already pointed out on this thread.
There are a few different models; they vary in DC amperage output. If you want to charge your batteries the quickest, get the 80 amp version.
If you want ease of installation, get a direct-fit model that will swap right out in a matter of minutes...these models are just as good, but don't supply QUITE as much amperage to charge your batteries as fast.
But, the availability of that full 80 amps is limited anyway by the size of your
12v and GND cables going to your batteries from the converter so this may be a moot point. It is highly doubtful that Airstream used premium super heavy gauge converter-to-battery wiring in a Classic anyway. They cheapened out on the converter itself and the fake wood, so there is absolutely NO reason to believe they would splurge for high quality wiring.
Get the Intellipower that is a drop-in replacement and be done with it.