Just make sure you use the exact same capacity battery and preferably the same name brand. I am assuming since it is thor it is probably a Harris battery. It is best to only add new batteries to new batteries as when you add a new to an old the old will draw down the new to try and equalize the voltage. If your current batteries are weak and that is the reason you are considering adding a new one might want to think about buying all new. Granted people add new to old all the time and you will see a benefit from it but it will most likely shorten the life span of the new, depends on how tired the other batteries are. My harris batteries are poor and I just got the coach in March. I will just replace all of them on mine.
As for connecting, just keep all the positives connected together and negatives together. So you will have to run a positive cable from the new to the old and same with negative.
2015 Thor A.C.E. 29.3
2014 Honda CRV Toad
Ohio FMCA 317123
Camping World Indianapolis, worst dealer EVER experienced