I dont know where you guys get the Idea that a cars ECU / PCM / BCM computers need to be powered to retain memory. We quite regularly pull ECM/ PCM /BCM s and send them for testing or repair. no power. they work just fine when they return. Same when replacing toasted engines, clutches, Transmissions etc. about the only thing you need to do after reconection is set the clock and code the radio.
Ive spent nearly 25 yrs as a diesel mechanic and another 6 or so working on cars. learnt a far bit about single and multi battery systems over the yrs as well as starter motors Alternators etc.
Bottom line is if you are going remote then get your battery and charging /starting system load tested before you leave and replace anything that doesnt come up trumps.
Reminds me I must go and take the batteries out of the boat as wont be seeing any use this yr.