I am surprised they still make a 3 way refrigerator. As you have found out, the 12v eats the heck out of the battery. It would only be needed if you run out of propane and gasoline (to run the generator when no shore power is available).
Just my own experience, but in over 20 years of RV camping, I have never had a time where I wished I could run the refrigerator via 12v.
Don