I have a tip for testing your water system with air vs water for the first time after winter.
I did some research and show no other similar post. For us up north like South Dakota have had below 0 temps for about 3 weeks now.
I will do this to make sure winterizing was a success.
Take rv blow out plug, load up system with air pressure(I believe about 35-40 PSI), wait about 24 hours(give at least 24 hours in case of slow leak). Open facet and see if any air comes out. If it does you should be safe your system made it through the winter.