I am no auto elec but I would suggest that running a fridge off your cranking battery and running the car for 15 minutes would simply shorten the life of the battery dramatically. Batteries dont like running under 50% of their capacity and the last 20% of the charge up time takes just as long, if not longer than the first 80%. Take your car for a drive if you want to charge them up, this way your alternator should be putting more amps out and you can enjopy the scenery as well. I reckon a car just idling is nothing more than an over sized generator anyway.
I have just gone through this exercise due to the fact that my 55amp deep cycle doesnt have the kahunas to run the fridge for any real length of time. I compared using generators, solar and dumping another 2 deep cycle batteries and it was the later option I have settled on. I am going to use my existing battery isolater connected to 2 AGM deep cycle batteries and the 55amp will be disconnected from the system altogether. I will charge this one from the mains before I leave home and use it to run my camping lights only.
Scols, the only thing I would suggest is to go straight to an reputable auto elec and not one of the main stream 4wd shops. If you decide on a battery system, have them work out your usage on a worst case scenario. At least this way, you know you'll have plenty in reserve.
Check out http://www.fridge-and-solar.net/
this website is easy to read and understand. Hope it helps.
Please sir, Can I have some more? Time off that is.
"Hard at it"
Fruit Bat Falls, Cape York