I have a deep cell 12 volt battery in my fifth wheel. I also have a spare 12 volt deep cell battery. If I connect them in unison will I have twice as much stand by power for boondocking?
A single group 24, even a group 27 battery is not much. Best to do the research & pack as much amperage in as space permits. This could mean a couple of group 27s, a pair of group 31s, a single 4D or even a single 8D.
Generally speaking, the fewer connections the better, hence the 4D or 8D.