I would take the money you might spend on extra batteries and invest it in an appropriately sized solar system for your dual 6 V system. Two good 6 V (I'm assuming golf cart, or similar) and an adequate solar system should be enough to carry you for several weeks at a time, unless you have extremely high power needs.
There are not a lot of places IN a trailer to put batteries. If you go to sealed AGMs then you have more options. But you still have to consider the weight. Somewhere like under a bed or dinette, possibly with reinforcements to the floor or frame, is a reasonable inside location.
If you don't use sealed batteries, you could weld a support system to the frame to keep them outside.