I previously used extenders until one leaked and caused a tire to go flat while parked. It could have caused some major damage, but I caught it before driving off.
The commercial tire place recommended I not use the extenders saying they leak too frequently to be reliable. Apparently, the extenders rub on the outside tire wheel where they go thru the hole in the wheel and that's where they fail.
Instead, get a commercial type pressure gauge that allows you to reach in and check or fill a tire without using an extender. They are very inexpensive.
Holiday Rambler Navigator DP, Hummer, and Honda VT1100C Shadow