The RM35 nominclature is, I believe for a 3.5 cu.ft. fridge like in our '64 GT. Your Tradewind would have had a larger one, as each incremental size of trailer came with a incremental sized fridge "back in the day".
Yes, firdges of this era can be found at RV salvage yards, however, most will be in a sorry state of disrepair. If you do find the right one, if can sometimes be fixed, however it is costly to do so.
Your best bet IMO is going to be to place an ad in our "wanted" section of the AirForums classifieds. Maybe someone with a similar age trailer will be stripping their guts out for a remodel and will have a donor for you.
Good luck in you quest!