Welcome to the forums. You will find a host of knowledge on here.
To your immediate question: IMHO a 32 year old a/c is toast. Forget it. But, the radius's of both trailers are the same, so if your front and rear bulkheads are messed up, you should be able to use the ones from the Land Yacht.
I had a '77 Excella 500 31 footer. It had a frame that was sized too small and flexed too much. My trailer exhibited the dreaded "sag" and "separation". If you do searches on those two words on here, you will find volumes of info on it. The good news is that your shorter trailer should not have these problems.
Realistically, I would not advise you to buy the other trailer as a source of parts for your current rig unless you need windows, etc. and you can buy the 31 footer for next to nothing.
Interior parts can be anything you want. I personally would not waste my time replacing the interior parts of my 32 year old trailer with interior parts from another 32 year old trailer. I would make new parts from scratch combined with ordering brand new stuff for my Argosy.
That being said, you might be able to get a shower or something out of the Land Yacht. But really, you'd be better off to fabricate something brand new than to try to strip stuff out of that one. Unless of course you could get that one for such a low price that you couldn't go wrong.
IMHO, I think you should make all new stuff for your Argosy. My grandad is making all new stuff for his '58 17' Traveler and it's turning out great.
Best of luck,