I think yours and other advice was sound - I think part of the issue was the fact that the batteries were connected together and then run to the starter, etc. I say this because when I connected a voltmeter, I most often was scratching at the post clamps and also at a hinge in the connection, where the wire connecting both batteries connected to another wire leading to the starter, etc. I believe that when I was touching the voltmeter leads to these points, I was getting a false reading, a reading that didn't indicate the poor connection to the battery posts.
I realized this when I tried to charge the batteries last night. The charger wasn't reading anything when I connected the alligator clips to the post clamps, unless I really wiggled them around, and then the reading was only brief before it disappeared. After I took the Dremel to the post clamps and made everything shiny and clean, the charger had a good reading.
I don't believe the new battery totally discharged, but I do think that after trying to start the truck many, many times, it drew current from the batteries, and that current was never replaced (or only a little bit) by the alternator.
I also think the that everyone is right, that I should've bought two new batteries, and will likely buy another new one to replace the second old battery.
So I guess one truck is better than none, but it sure makes pulling two trailers difficult...
I'll update this thread as the need arises. In the meantime, thanks to everyone for taking the time to share your insights with me on this situation. It made me realize that I don't know anywhere near as much as I'd like when it comes to diagnosing truck problems. I guess just being a landscaper isn't enough.
