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?
The only draw back is when charging. If the batteries are not like batteies and about the same age then they may charge different. They may also tend to balance over both batteries and this usually brings the newest battery down to the life term of the older battery.
Most folks like to have the two batteries the same make and age to get the best performance out of them...