I just remember my brother in law's experience with his PVC water lines under his house. He had drained all lines, but where they sagged and water pooled, when a surprisingly strong freeze came along, and caused all sorts of damage! You'd think it would push the water along to the dry parts of the line as it froze and expanded...but Oh, NO! The water expanded and blew holes in all the low parts of the water line all under that house. He had to replace the complete system.
I don't wish a surprise hard freeze to cause that in our RV...so we drain, blow out the open lines with an air hose and add antifreeze as well.
Just best to go with a little "insurance"!!
Happy days all!