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