I just purchased a 2014 LTV Free Spirit 22, built on a MB Sprinter 3500. This does not come with TPMS. As it is nearly impossible to check the air pressure a dually, it worried me that the tire pressures were right each time I jumped in the van.
To solve the problem, I ordered a DuallyValve kit from YourTireSupply.com for $120. This kit consists of rigid chrome metal valve stems that replace the OEM rubber stems and extend them to a place where they can be reached with the wheel covers in place. I also ordered a Invtek Solar Power RV TPMS Tire Pressure Monitoring System with 6 Wireless External Sensors kit from Amazon for $130. I chose this TMPS because the display is solar powered and displays air pressure and temp for all 6 tires.
I replaced my OEM Continental VancoFourSeason tires with Michelin Agilis CrossClimate (another story in itself, but WHAT a difference- The van is 50% quieter now!!), and while the tire shop had the tires off, they installed the DuallyValve stems for me.
Everything works perfectly. Once the van rolls a few feet, the sensors wake up and send the signal to the display so I know immediately what the tire pressure is. If pressure drops while on the road, the display warns me of the change. The DuallyValve stems make it easy to add air when needed without removing the wheel covers. The display sits on the dashboard to the left of the instruments, next to the “A” pillar.
I highly recommend these products.