If there is room for two batteries on the tongue then you should go with two 6 volts. The 6 volts from Costco or Sams club are they best bang for the buck. They are the same size as the standard 12 volt that trailers come with only they are an inch taller. To use two 6 volt batteries you only need to add one wire about a foot long into what you already have. You can buy this wire pre-made at Walmart or Autozone or any other auto parts store.
The 6 volt batteries are true deep cycle batteries that can withstand more uses or cycles of draining then recharging without damage than a standard 12 volt marine "deep cycle" that you can buy at the same parts stores.
