Batteries have a rating for how many cycles they are good for. The deeper they are discharged, the less cycles they will do. True deep cycle batteries might go 3000 cycles if they are only taken down 10% (90% full) and then recharged. Taken down to 50% regularly they might only do 1000 cycles before they are toast. Taken to fully dead they might be toast after as little as 100 cycles.
Engine start batteries are designed to be taken down 5% or so (start the engine) and then recharged. They can do that thousands of times. But take an engine start battery down to fully discharged and it might survive no more than a dozen or so cycles.
So if you plan to run your engine battery dead and then jump start it - you're going to ruin it - quickly. Even if you only drain it 50% regularly, you are still going to ruin it - just not as quickly. Engine batteries are not designed for that.
Reserve capacity is a red herring with start batteries - they have very thin plates inside designed to dump a lot of amps in a hurry, i.e., starting an engine. Cranking amps is important. If you're looking at the reserve capacity of an engine start battery - the you are probably doing something that really should have a deep cycle battery instead.
Your fridge might do a good job of keeping food cool overnight, so it's possible you could just unplug it and not deal with the problem.
Failing that, I would run it off the portable battery pack and stay strictly away from the engine battery. That power pack probably says to only charge for 2 hours from 12v because some people will plug it into their lighter and charge it, and kill their engine battery doing so...
But even if there is some electronic reason why it can't charge more than 2 hours from 12v you could plug it into an inverter and charge it that way.
Best to get another battery to run the fridge. You could hook up a lowbuck inverter to your engine battery, and plug a lowbuck battery charger into that to charge the aux battery and have a quick and dirty dual battery setup. Just remember to switch off the inverter when the engine is off.
For that matter you could hack open a powerstation jumpstarter doohicky and unhook the battery and hook up a cheap rv/marine battery which, while not really deep cycle is still better than using the engine battery.
Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
Previous wheelers: 41 Willys|78 FJ40|78 Bronco|84 Bronco|74 Ramcharger|78 Ramcharger|79 D150 PowerWagon|77 D100|79 D400 dually, converted to 4WD, utility bed, 10' Lance|75 Westy|69 Scout, RHD|bunch of others|bunch of bikes|couple of boats|couple of motorhomes|blah blah|so what|not my idea|just doin' what I'm told|wank wank|this space for rent|candy is dandy|but liquor is quicker