Ok, I give. What is the secret to removing the valve stem cap that points inward on the duals?! I simply cannot get my hand through the hole in the rim. I have to draft one of my daughters each time I want to check the tire pressure!
They can be plugged from the outside like a light vehicle tire, but most tire shops do not recommend this. The proper repair is to plug the hole and then patch it from the inside or use one of these:
Uni-Seal Plug/Patch Combination Units
I used to plug truck tires all of the time when I managed a drilling rig moving company as sometimes you just don't have time to dismount and "properly" repair a small nail hole. We found that if a plug wasn't going to hold it would fail before we got the tire blown back up to full pressure. So for the relatively light use we put on the drive tires of our coaches I would do it and not worry. Steering tires I would plug to get out of a bind and get properly repaired as soon as possible.
Worth every penny for peace of mind for those of us that are neurotic about checking the tires. You still need to do a visual walk around every time you stop like normal, but no more crawling around with the gauge all the time, or worrying about it if you didn't. Also we have discussed many times here that saving just one tire by catching a repairable leak vs. the tire blowing out will pay for the system.