Tagging on to this old thread as others may find it useful...a similar thing happened to me just last week with my '07 2100.
I just had a leaf spring added to my PC (wow, did that make her ride much better!). A day or two later, doing my pre-trip checks, I noticed my inner right rear tire was flat. Took it to the tire shop thinking it had a leak, and they couldn't find anything, put it back together and it held air tight. They checked the extenders/etc but found nothing.
They noted that they do not like the extenders, said they caused a lot of leaks. My theory is when the suspension guys put the wheels back on, one of the extenders leaked.
Another week later, checking the air on my wheels before a trip the other side rear wheels air line extender had a hissing sound after taking my tire gauge off the stem. I monkeyed with the stem best I could but was able to put the cap back on and with the o-ring in it, it held for our weekend outing. Phew.
Anyhow, will see if the valve stem tool (Ordered http://www.amazon.com/gp/product/B009EPNNHE/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A1THAZDOWP300U
from Amazon) does the job. If nothing else, it will be a good thing to have in my home tool box.
Hope you are all enjoying your summer!