If you start out with METAL valves, and add Extenders that are solidly mounted to take the stresses off, you are usually/probably OK.
Even better if they are "AirLess" extenders. With those, there's no air pressure "living" in them between checking/airing. When you push a gauge/chuck onto an AirLess, you actually push on the end of an internal cable that presses the core in the actual valve down at the wheel.
Most extenders are not AirLess. When you screw it on, it depresses the core at the rim and carries pressure out to its own core. Now you're relying on the tightness at the actual tire valve, plus the condition/integrity of the rest of the extender, finally its own core and cap.
Just don't attach the outer end of the extender(s) to the wheel simulator/hubcap. If that comes off the rim you either keep the cover to beat up the coach, or the extender breaks and lets the air out of the tire.
Yes extenders work. The better ones, well installed, are OK. They're just not the Gold Standard. Those come, at a price, from Borg and Tire-Man
