Add third 12V battery in parallel to travel trailer?

ScottyP5947 wrote:
They are combo start/deep cycle batts so not really the best for the job.

Actually East Penn does refer to their DEKA DC series as "deep cycle", the DM being their marine start / deep cycle series. While a pair of 12 volt G27s in parallel do offer less capacity than a pair of 6 volt GC-2s in series a pair of larger G31s offer very similar ratings to the GC-2s.