I recently purchased a RV and it has no auxillary batteries in it, it's a 12V system and has a place for 3 auxillary batteries. I know that they are designed for deep cycle batteries and from what I've read they need to be hooked up in parallel which increases the amperage but the voltage remains 12V. Am I on the right track?

 

yes you are on the right track..
batteries in series = voltage increase/same capacity
batteries in parallel = same voltage/capacity increase