OP here...
I did get my 'Service Address' changed to the Thermal, CA area easily enough calling 800-769-4635 where I talked to a helpful guy who sounded American. Called during regular business hours. Only took a few seconds for him to change my service address, and bingo I was watching local NBC with all the other national networks available too.
A few days later, it was time to head for Mexico.
When I got settled here in San Felipe, Mexico, and got the satellite working here, 213 miles south of the US border, I was flipping through channels and passed 391-399 and discover, hey! they're working! So the call I had missed from DirecTv while traveling was probably about the DNS service, despite my FAX telling them to cancel the service before it began.
So all is good for now, and it's a surprise that I'm receiving both the local channels from Thermal, (I had read that the DTV spot beams only worked around 50-100 miles radius from the service address zip code), and now the national networks.
One of my Mexican traveling buddies says that I should be able to get DTV all the way down to Cabo San Lucas (the southern tip of Baja). Which is my destination this trip. So I'm going to go ahead and keep paying that extra $20/mo for the national networks at least until my Mexican travel is over.
Thanks for everyone's advice! Especially the info about 'Service Address'. When I get back to the US, that'll save me $20/mo.
Happy camping!