LLoyd, I guess they're technically called "Valve Stem Extension Stabilizers" and can be purchased in a variety of sizes and shapes to fit your specific application.
I'm surprised that your wheels don't already have them. Do you have a Freightliner chassis with aluminum wheels? If so, many FCCC coaches that have aluminum wheels have a hole pattern that would use a rubber stabilizer in the shape like the one in the image attached below ...but many motorhomes use wheels with round or oblong holes too and they make the valve stem extension stabilizers in a variety of sizes and shapes to fit those holes.
If you go to this link, you can see just how many sizes are available: CLICK HERE
I take mine off each time I clean the wheels and clean them separately once they are off. I then just pop them back on.