What if you got a large (950 or 1150) jump pack and wired it inline on a switched connection between the fuse box and cooler? When the Jeep is off it will work off the jump pack and when you run the engine it charges the jump pack battery?
My big fridge/freezer has a different compressor than the smaller one and seems to need a higher voltage than a battery not being assisted by an alternator can provide.
What's wrong with using the jeep as the generator?
12-14 hours on the surf using a 3.8 liter V6 guzzler or a 50 cc single cylinder Subaru generator? You are right. It is a tough decision.