You are going to need some kind of isolator between an existing bank of batteries and a new set. If you just wire them in, you get some kind of yo-yo effect where the power flows back and forth between the new set and the old set due to the differences in charge characteristics. This causes premature failure due to plate wear. And that is assuming you are buying the exact same batteries (make and model) you have for the other 4. Anyone correct me if I am wrong but that is what I have always been told. I assume a battery isolator (like you would use to isolate the TV batterie(s) from the RV batteries) would do the trick but it would induce small inefficiencies too.
2017 Wildcat Maxx 262RGX with 4x100w solar panels