Not sure what you mean, but the Firestik (and many other antennas) will use a 3/8"-24 screw-in base. If you're looking at a 12" flexible rubber antenna, it's probably designed for a handheld device and uses a BNC connector. Without more info, it's hard to tell.
One other thing to keep in mind, though. Motorhomes are notoriously difficult platforms for decent radio operation, in part because the nature of their construction does not provide a "ground plane" for the antenna. The Firestik is specifically designed to mitigate this problem by using the base and coax as a substitute ground plane. Using any antenna that requires a ground plane on a motorhome, without modification, will almost certainly give you such abysmal performance that you might be better off with two tin cans and a very long string.
๐If you can get some further information, post it here and I can maybe be more helpful.
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K4EAK
2013 Thor ACE 30.1