surferdave - If you want a Lifeline battery I can get you one in Costa Mesa. Not necessarily "affordable" but they are very good batteries. Whatever you get should be AGM to handle Deep Cycle.
Grp 65 will be about 65Ah (Pep Boys online when on sale about $195)
Grp 27 about 100Ah
Grp 31 about 105 to 110 Ah
The battery box you picked up, if it is a match to what you have, will pretty much only handle a Group 65. Some have said they can fit a Group 27, but I have not had success with that. You can modify the box to accept a Group 31 (or Group 27) by cutting out the face and allowing space for the battery. The AGM's can mount on their sides so that is beneficial.
You say your fridge is only drawing .77ah - that seems very low for an average on/off net draw per hour. Normally they are closer to 2 to 2.5 effective ???
Even if your .77 is correct you are looking at 20Ah/day. Using the 50% limit for draw down, you are at 2 days for the Grp 27 or 31.
Are you considering Solar??
Beastie 3: 2002 7.3 EB Cargo: Agile TTB, CCV Mid Top, Custom Walk Through, Lots of stuff added. BlingMyRig.com