Again, on a personal note, i prep my coolbox before loading it, i leave it switched on overnight at home via the mains,with a couple of those frozen freezer block slabs in the base for good measure. Then upon loading time, the coolbox is switched off, the freezer blocks left in the bottom (two fit snugly alongside each other in the bottom of the coolbox) Then the coolbox is loaded up deftly to minimise temp loss. The coolbox is then loaded into the bongo, engine started, then plugged in via the ciggy socket to maintain a cool temp. Upon reaching the destination, i unplug the coolbox, and won't open the lid unnecessarily. When i DO open the lid, i switch the coolbox back on again for half an hour or so to compensate for the lid being opened.
This method minimises power loss from the L/B i reckon! Dandy? The white deposits on the battery terminals... Isn't that common when the terminals are dry? I've had it happen in the past with other cars, and used the excellent method you described earlier to remove the deposits(slightly cooled boiled water, Coke works too
) Then applied a generous amount of copperslip to lubricate the terminals, which stopped the white deposits from returning, or was i merely masking another problem?
) works better - maybe it sealed the post 
