Add third 12V battery in parallel to travel trailer?

Sam Spade, Wouldn't adding another battery add the amount of available power? If you had one battery at , say, 100--and added another battery at, say, 100---wouldn't that add up to about 200? Now add another battery at, say 100--wouldn't that add up to 300 total?
Not saying this is something I would actually do, but I have explored the possibility of a third 12 volt battery in my battery bank.
Just questioning....