OK, I've actually got it working, but I don't know why, so that scares me, so I figured I should ask.
So I downloaded my XML file and it has the proper size:
"wget -O /path/to/tv.xml https://internet_location_url/epgfile.xml"
I made sure that xmltv was installed on my system and enabled the external grabber on the "EPG Grabber Modules" tab
Then I ran:
"cat /path/to/tv.xml | socat - UNIX-CONNECT:/path/to/xmltv.sock"
I go to the "Configuration | Channel/EPG | EPG Grabber Channels" Tab, see that the "Module" column is filled with XMLTV and the path to the xmltv.sock is correct. I make sure to enable all of the modules. They all show to be updated to the time I ran the above "cat" line.
I'm more thrilled because then I go to the "Configuration | Channel/EPG | Channels" Tab, and the "EPG Source" column is filled with the correct entries!
Then I go to the "Electronic Program Guide" tab and .... there is nothing there. Empty. Nada.
Huh?
So I go back to the "Configuration | Channel/EPG | EPG Grabber Channels" Tab and check the "once per auto channel" box for one of the channels, and lo and behold, it shows up in the "Electronic Program Guide" tab. I go back and do it for all of the channels and they all show up in that tab.
OK, what did I do, and since it's only "once per channel" will my EPG not update in the future when I update my epg.xml file and send it to xmltv.sock?
TIA in advance