I see no possible reason why it would void the warranty. You're replacing a 12V battery with a bigger 12V battery that just happens to be packaged in two separate units for (relative) ease in handling. Before getting the two golf cart batteries, though, measure to make sure they will fit; they're a bit taller than the standard 12V battery, and sometimes there's not enough height even if the battery compartment/box otherwise has room for two batteries. Assuming there's space, it's a relatively simple and cost-effective way to get a good bit more battery capacity.
That said, do your parents feel any need for more battery power, or did they find the old battery was more than sufficient for their needs/wants? How often and for how long and in what environment do they camp without electricity? What are their electrical requirements when they do? What sort of a charging setup do they have? Possibly money that could be spent upgrading the battery bank would be better spent on a modest solar installation or an upgraded converter/charger. (That's not to suggest that the dead battery should not be replaced, of course!)
