As it turns out, it was the alternator. Was wishful thinking that it would only be the VR I guess! Just a strange coincidence that I ran the battery down by leaving the key on and this happened - but I suppose that was the last straw.
When we took the voltage regulator off to check the brushes, one had actually worn so far down that the spring was rubbing on the rotor, and it was scored (235,000 miles on the truck, you'd think they'd at least last a quarter million ...
). Can't believe the truck never had a problem or indicated anything was wrong before I ran it dead! It wouldn't have been long.
Thankfully we've got a great automotive electrical shop nearby (Inland Auto Electric in Ellensburg, WA if anyone in that area needs some really quality work done), and she had a rebuilt alt on the shelf. So we're back up and running and charging away.
Thanks for your input!!