PROBLEM SOLVED. I must tell you that all my Volt Meter investigation did not solve the problem; it was solved with DIVINE INTERVENTION. After about 3 hrs of Volt Meter work, I decided to start at the beginning, at the batteries, check these new ones (manufactured Feb., 2013) under a load. I searched all local stores for a good load tester; nothing. Ordered a Midtronics one to arrive on Tues. After an hour of thinking about the truck, I knew I could not wait until Tues. so I loaded those monster batteries in my car for a trip to the auto parts store for testing. As I lifted the one closest to the firewall out and started to hobble away, I noticed the bare end of a large, red jacketed wire pointed straight out of the engine compartment. See Photo 1. It was the wire from the Neg. side of that battery; the 12V supply for the diamond-shaped 12V block near the master cylinder that Warthog had earlier said to check and I did. It was 12V; it was always 12V each of the probably 6 times I checked today. However, it was NOT the 12.8V measured at the battery. These 12 volts simply did nothing under even a minor load. It took the unseen hand of the Devine to let that wire slide out of the poorly crimped cooper connector at the battery terminal
(see photo). Got my trusty, mega power crimper to do the job right, shined everything up, attached battery connections, tried lights and wha lah; light. Timidly started truck, got starter action. After about 10 off/on starter tries, the engine roared to life. Big bonus; no leaks detected at the IP; will check further tomorrow in the sun.
I want to thank all of you who took the time to offer advice. It was all sincerely appreciated.