I have experienced two flat tires on two different MBS chassis. Both of the flats were from broken valve stems. The latest was on I 80 in CA. Fortunately a passing driver signaled that something was wrong. I pulled off the freeway to a safe area and changed the tire which was damaged beyond repair. No damage to the fender well. The tire repair shop said the valve stem broke from stress caused by the valve stem extender. I had a heavy duty metal stem installed on the new tire. Suggest if you experience a stem failure that you use a new metal stems.This failure occurred at 12k miles. I'll be replacing the remaining stems in the near future. Changing the tire is a pain but the jack provided will lift the uunit. I will be considering a TPMS. Safe travels.