The basic info is to add in series (pos to negative) if you want to double the voltage. Most people do this to make two 6 volt batteries into one 12 volt system. Be careful putting two 12 volts in series that will yield 24 volts - good for some fire trucks and army trucks.
Most people just want batteries to last longer so they use Parallel - Pos to pos and neg to neg. The RV red lead goes to battery 1 positive, battery 1 positive goes to battery 2 positive. Battery 1 negative goes to battery 2 negative and the Black RV lead comes off of battery 2 negative. The RV voltage will stay at 12 volts but you have double the current capacity and therefore the battery will last twice as long with the same draw.
If you have a normal 210 amp hour battery by adding a second 12 volt battery in parallel, you double the current capacity to 420 amps. A 1 amp draw will last 210 hours in a single battery and last 420 hours with dual batteries. Great for dry camping.
Bob & Sue Rowan
2012 Coachmen Mirada 29DS (SOLD)
2015 Keystone Bullet Travel Trailer
2016 F-150 Lariat with Auto Trailer Backup
Dog = Morki (Yorkie-Maltese) - Zoe