Is your RM 75 non fixable? If it is, I'd refurb it and use it another 40 years. The good thing about the older refers is they don't need 12V
. All the important parts are pretty available for the RM 75. They are built to last. I have seen many brand new RV fridges give up after one or two seasons. If you want a really good new unit, I'd recommend a Danfoss type.
A little measuring and you could calculate the ft/3 of the RM 75. I know it will hold 30 cans of beer, half gallon of milk, pound of bacon and a dozen eggs.
I think you probably get more volume from newer units