And, No, I did not put that petcock in the tank. It was there when I bought the rig. looking back, I should have realized that was going to end up being a problem. But that is hindsight for you.So, lesson learned. The petcock, apparently had become clogged from sediment in the tank, and had never drained. (that split was in the top of the tank, BTW)
Putting a petcock in the drain plug proved to be a really bad idea, and relying on it for draining the tank was even a "more bad" idea.
The tank actually had a lot of sediment. Probably years of poor maintenance and improper draining from using that petcock, instead of pulling the plug. But if the thing lasts another 20 years, I won't have much to complain about. Hell, by then, I probably won't even remember where I parked it.