Thanks poppy and ned, I've seen those. I'm wanting to use solid brass extenstions that are permanent, not screw on. And, They come with a rubber "plug" that fits into the simulator "hand hole" to secure the stem. With these you can use TPMS systems that screw on to the end of the stem without worry of losing them.
I thought maybe someone knew the hole sizing. I'll go out to my RV at the storage place and measure again.
Here's a photo of type of stem I'm looking at - you need the correct hole sizing for that rubber stabilizer plug: