My guess ( and just a guess is your furnace is an NT-22). It should be stamped on the data plate on the case. If it an NT22, best bet would be to scrap it. There is a newer model Suburban that can be fitted into the NT22 case, lots of stuff on the forums about how to do it and why the NT22 can be dangerous even if it works.
If your A/C is an old Bay Breeze unit there are charge ports on the top under the cover where the refrigerate lines go down into the trailer ( a spot for leaks as well so if you have the cover off take a tube of Vulkem up with you to fill any gaps in the gasket). Shroud comes off and then the top cover. I think it is R-22 but not sure. Unless you have the equipment I'd let a pro look at it. Caution, when on top only step on the area over a rib (identified by the cross body row of rivets. The long rows of rivets about 2/3 of the way up are NOT on ribs.. Be careful or you can dent the top.
Tom W has a good web page with pics and details of what was needed to bring his old unit back to life. Short version is that they can be fixed but will be cheaper to replace with a new unit if your compressor is shot.
Here is the site: 1967 Airstream Overlander