Here's my suggestion: Do yourself a favor, save yourself the risk of air leaks and skip any kind of extensions altogether. I know many use them for years without a problem, but IMO they are just a leak waiting to happen. I had extensions less than a year before one of them got hit by road debris, knocked loose and let the tire go flat.
Save yourself the risk of that ever happening, and get you a set of
Dually Valves and be done with it. Complete, solid valve stem replacements, not extensions. Can check, refill with air all of your tires (inner and outer) very easily, without any special gauge, and in many cases without having to remove wheel covers.
I just put these on my MH, and really like them. An easy, simple, fool-proof solution to an easy, simple problem. Thats what I like. :)