Boot 12V Socket Power
I've been thinking a bit more re your freezer supply. Not knowing what it's consumption is, makes it a bit hit and miss.
I think I'd be tempted to run a separate supply direct from the battery under the bonnet via, of course, a fuse near to the battery terminal. It could be that the volt-drop on the existing socket wire will be enough to trigger the automatic cut-out in the freezer. I'd use a nice thick wire in order to minimise volt-drop. Bit of a pain but should yield a good, reliable result with little mods required to vehicle electrics. Hope this helps.