GC2 batteries is the way to go with that scenario of RVing, if you have the space I certainly would make the effort to get a couple new boxes and whatever simple wiring modifications that are required.
We started out with a pair of Interstate group24 rv/marine batteries which served their purpose as well as can be expected, but when the time came to replace them we went with a pair of Interstate GC2 batteries from Samsclub.
They took about the same space although there are a bit taller. Grabbed a couple of battery boxes online for cheap, new wires from the auto parts store and we have never looked back.
Obviously, if you don't have enough clearance for the taller golfcart batteries you might be headed for the option your RV shop presented, which really is not all that bad. Maybe peruse Samsclub/Costco as pricing will be much better.