The water lines in your trailer are very safe as long as you have heat on. The lines that froze in my rig were in my motor home.
Generally when we travel we don't have our heat on at night, we use an electric blanket at night. However, though the rig gets cold, we have not had the internal temperature drop below freezing though the outside temperature has been well below freezing. The Sunline's pipes are relatively safe.
The outside hose is another matter. We always disconnect if there's any chance of overnight freezing. We have had a hose destroyed by freezing.
You are correct about the toilet; it does sit on top of the black tank. It is connected to the water line at the rear of the toilet. Water to the toilet is controlled by the first little lever on the right rear corner of the trailer. The bigger lever flushes it.
It's possible to download the manual for your toilet, actually this is true for practically every appliance in the trailer. I'll look later and see if I have the toilet manual.
We do carry chemicals with us for the toilet. We use TST RV toilet treatment and buy it at Walmarts. We use it probably every other dump period.
The use of chemical treatments is a sticky subject. People use all kinds of 'stuff' and I don't have a real basis for what I do.
Norm Milliard