I use extenders, but in a different way. On our old Four Winds/Ford, it had the tiny "hand" holes (The ford "engineer" who came up with these should be made to check every RV in the world) so I had to come up with another method without using permanently installed extenders.
Here's the tools:
Short permanent extension on outer dual - I had rubber valves, so I added a small support wire:
View through the "hand hole" to the inner dual schrader valve:
Using the length of 11/32" rubber hose, remove the cap:
Using the long neck dual foot valve gauge, check the inner dual:
If the tire needs air, screw on the extension. The female end is fixed, so it can be rotated using the hose:
Attach your compressor and fill to the desired pressure + a pound - it helps to have a screw fitting, rather than the press-on style:
I add a pound extra to compensate for the lost pressure when unscrewing the compressor/extension fittings.
I put the same style extension on the outer dual of our new Sunseeker and will use the same process. I have the new Pacific Dualies wheel liners, and the hand-holes are about half of the size of the original GM hand-holes:
Hope that helps!