Do not use extensions, evil things. Get valve stems that are long enough.
We had ours installed at Les Schwab in Junction City Oregon. To far to go from NC, but here is the web page that describes what they install:
Bob Dickman Tire Center - DuallyValves™
You should be able to get these locally or on the Internet.
In our situation we had a PressurePro system installed, so it was easy to determine if a tire was low. But in order to add air we had to remove the wheel liner. The stock tire stems and extensions made it impossible to get some of the PressurePro sensors on and off.
After stewing on this for a while and imposing on the local Les Schwab tire dealership, they recommended I talk to the Junction City Les Schwab. I did and eventually the planets aligned and we took the RV in to have the tire stems installed. Now I can add air myself.
BTW: The Walker Rd Les Schwab has checked the air in my RV tires 3 times for free, spending almost 45 minutes each time.