2 6V batteries wired in series will give you 12 V. It is a common swap to use 6 V golf cart batteries wired in series to provide 12 V power. I did it on my first trailer years ago and found I got longer life from that setup. If your batteries are toast then you have discovered why the 12 V isn't working. You can either replace the 2 6V batteries and make sure they are wired in series or buy 1 12 V OR 2 12 V wired in parallel with each other. 2 batteries will not drain as quickly as one. If you do use 2 batteries, make sure they are both in the same condition,meaning both new, not one new and one old.
