Problem solved! It seems I assumed myself into trouble!
The rest of the story:
I'll refer to the auto ChangeOver Regulator as my #1 regulator (streetside), and the curbside regulator as my #2 regulator.
Back in May, my #2 regulator started leaking thru its vent hole. I took it out and went to an Ace Hardware store in Camp Verde, AZ looking for a replacement. The salesman showed me a regulator that was labeled "For RV's". It looked identical to the old one (same silver color, same basic shape) except it had a large vent instead of a small hole, and the “Out” port was larger. The salesman solved that problem by selling me an adapter.
Important Fact: The new regulator was a low pressure regulator and the old one was a high pressure (30 psi) regulator which is what I should have bought.
Off I go - fat, dumb, and happy. I install the new regulator and everything worked fine with no leak. Keep in mind that the auto regulator was pointing to the #1 tank.
Fast forward four months to September when the #1 tank had emptied, the auto regulator had switched over to the #2 tank, and a while later the #2 tank reached half empty. (I still do not understand how the auto switchover happened with the #2 regulator outputting 11 column-inches of pressure, but it did and all LP appliances worked fine including the furnace.)
When the #2 got to half empty, I shutoff the #1 tank and pointed the auto regulator to the #2 tank. I took the empty #1 tank and got it refilled. After hooking it up, when I opened the valve on it, gas poured out of the #2 regulator’s vent. This gets us to when I started to troubleshoot the issue and made my original post.
Note: I must have overlooked Old Biscuit's post as I see today that the answer was there all along. ****!
Since the responses to another but similar post pointed one or more fingers at just about all components in the LP supply system, I started by replacing the #2 (low pressure regulator), then the auto changeover regulator, and was about to replace the #2 pigtail when nothing was making a difference.
This is when my web research stumbled onto an article on RV LP systems that mentioned a “high pressure regulator” that is used on 5th-wheels with “split tanks” (A tank on each side of the trailer.) The light finally came on, I installed a high-pressure regulator, and all is good now!
And my spare parts locker is closer to full as I now have 2 low-pressure regulators that are wrong for my trailer, and an auto changeover regulator that I don’t need (now).
As I said at the beginning, I shouldn’t have assumed that any “RV LP regulator” would be OK.