I go through San Miguel about twice a month. Here is what I've been able to figure out so far:
I've never heard any voice traffic and end up locking out this freq due to intermittent bursts of static.
I've heard the most traffic on this frequency and my scanner has found a CTCSS of 192.8. Transmissions are better received north of Las Vegas, at least as far as Wagon Mound. I too think this is dispatch, though I seem to recall not hearing field units often.
As for the other freqs you have listed, I do have them in my scanner and I believe I've heard voice traffic on them, but it has not been clearly received. During my last trip through the area, I could have sworn I heard NMSP on one of the freqs, but I can't confirm which one.
For reference, I'm doing this monitoring while driving along 1-25 and sometimes US-84. I'll be heading through there again this weekend so I'll make sure to keep a small notepad handy.