This question is answered in more detail on the product page of the Non-Isolated Smart DC DC Charger. Look at the "Additional Information" tab.
The only difference between the isolated and the non-isolated is that in the non-isolated the incoming and outgoing DC negative is shared. The non-isolated has only three terminals, an incoming positive, and outgoing positive, and a common negative. The isolated has four terminals because the incoming and outgoing negatives are isolated from each other
When you use this as a battery to battery charger in an RV the two batteries already share a common negative so there is no advantage to be gained using the isolated version. Isolation if really only an advantage if you are using this in a different context, to isolate sensitive electronics from electrical noise