I have the airless extenders on my E-450 and I am not a fan of them. The first set that came on the coach would not work. One of them on the second set came loose when the bracket attaching to wheel came off. I lost no air but it was bent to the point of not being usable.
I now have a simple $16 truck air gauge that has a 90 degree chuck that allows me to easily check the inner wheels. Likewise I put a 90 degree chuck on my tire fill hose. These allow me to service the inner wheels without removing the wheel simulators.
If you go to a commercial truck tire dealer, they have caps that you install so that you do not need valve cap covers. They keep the dirt out but allow you to use the tire gauge without removing them.
Save yourself $50 or $60 on extenders.