We offer this as a stand-alone item because not everyone wants to monitor the pressure in the spare tire. But most of us forget to check it before every trip. And then ... when you need the spare the most, it's low on air.
Using a spare tire that's low on air can be worse than having no spare at all. A tire that's low on air won't last long on the road, and could even cause damage to your Airstream if it breaks up.
With an extra sensor, you'll always know your spare tire is ready to go, just by glancing at the TST monitor on your dashboard. It's one less thing to worry about as you're getting ready for a great road trip!