Unless they changed, Forest River uses AirFlex airless extensions. Since they are airless, even if they crack, break, or get severed they shouldn't let the air out of the tire. They are a tube with a core down the middle. The core presses the valve stem at the tire (or, in the case of the outer dual tires on our 2012 Solera, the valve in the 135 degree metal extension attached to the tire valve) when filling the tire or checking the air.
I did experience air loss caused by the AirFlex extension moving around while driving and apparently working its core against the tire valve core. I have since tied up the extensions with wire ties to prevent.
As stated above, the tire valve stems - high pressure rubber stems on mine - seem to be the "weak link". Hard to find metal stems to replace them with - some have found "dually stems" that work. The Sprinter wheels are shaped in a way that prevents tightening most metal stems. The high pressure rubber stems are rated for the tire pressure and are what is recommended, but I experienced two valve failures (not catastrophic) with them. Tire shop said they had been "improperly installed". At my request, they found some metal stems that fit (barely) and replaced them.
I met Chuck Carvitto at a campground in central California - tire-man.iwvisp.com - who told me he had developed and sells metal valves for Sprinter duals. I had already found a place that put metal stems on mine - so I have not tried Chuck's devices.
2012 Solera "S"
Calif SF Bay Area