We have 10 sensors for our TST TPMS, 6 for the coach, and 4 for the toad. The toad's factory system cannon be fed to the coach. As said, the aftermarket sensors will not interfere with the factory system, nor will the factory system interfere with the aftermarket system.
It would be nice if there was a universal monitor for factory TPMS, but each manufacturer uses their own proprietary setup, and there are even differences among models, so a universal receiver is unlikely to be developed or be cost effective. On top of that, not all factory systems are radio based to begin with.