Ok. Thank you everybody. This is a good friendly community.
I figured it out. The black plastic flange at the end of the clear plastic tube that feeds water into the toilet was broken. This was a winter freezing problem.
Last fall I drained the water supply tank and the gray water tank and the black water tank. And pulled the 1"
plug on the water heater. Opened all the sink valves until no more water drained out. And all the water DOES drain out of the system, if you do all of the above, with one exception.
The water supply tube to the toilet comes out of the wall and bends downward and then back up to the "water supply module" on the back of the toilet. That forms a trap that does not drain. So the water supply module still had enough water in it to freeze.
I see this a design problem. A design flaw in fact. I will soon put a petcock valve in the low part of the U-shaped water supply hose, so it will drain. This was a learning experience. The new water supply module was in stock at a local RV supply center. It cost me $36 USA dollars. And an afternoon of work.
Next time I'll know better. Soon I will fix that water supply hose to it drains properly at winterization time. So the entire water system can be reliably drained.
I think that using air pressure to drive all water out of the system would have forced water from that low point. Agree it is desgin flaw, however.