As has been mentioned mixing different types and / or age batteries will prove to be far less than ideal. If cost is a concern, I would recommend getting two new 6v deep cycles and connecting them in series for 12V. They are available at places like Costco for under $100 ea, and you will end up with around 220Ah's capacity. That should allow you a couple days or more of refer run time, depending on what else is drawing on them. Assuming you are running the engine to charge them, adding a battery selector switch will require you to monitor the charge on the selected battery, and when fully charged, you will have to manually switch to the other battery. That's not idea while driving, and it will take longer to get both up to full charge. If you end up with two new 12v's, of the same type, using a battery switch with the ability to combine will work fine assuming you remember to combine them while driving then separate them once stopped.
Agile Ride improvement package