There ARE "High Pressure" "Rubber" Stems. They show Brass for a lot of their length, compared to standard rubber stems where brass shows only with the cap off. The High Pressure are OK only if there's nothing on them but the Cap. My brother has them on his one-ton pickup and they're fine.
Extenders/Extensions, NO NO NO!!! The added weight will vibrate and worry the rubber to failure. Get a solution that is
1. All Metal, and
2. So easy to use that you check air often, willingly, easily.
Camping World just started carrying Dually Valves, and at an unheard of low price. There may even be Coupons, Specials. Get a set (part number may be DL-1EC) and go to Discount. Have them install that set in the rear. And, replace the FRONT ones with METAL. Spare too! Two-inch if they have them. The usual 1-1/2" won't let you check and air them without and extension.
Get Air-Through Caps for all seven. I'll find you a link, but NAPA has them in blister packs of four. Amazon has them too. Look for V2B, Gator, Crocodile. Adding those to 1-1/2" metal valves lets you check and air Fronts and Spare. With those on every position, you can check tires as fast as you can get yourself in front of a wheel and push the gauge on.
