Never heard of a 776 EG2 but found a couple references online. I'm afraid you are going to have to wing it from the dimensions of the cut-out for the fridge.
Chances are it was 6 cubic ft, give or take.
Have you considered just putting a small apartment-size fridge in there? If you don't boondock and just travel to sites that have shore power available, that should work fine. Much cheaper too.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL