You would have to add a lot of antifreeze to protect a half full tank. I would just add water then dump. If you can't add water dump anyway then add water and rinse later when it warms up.
Class C, 2004/5 Four Winds Dutchman Express 28A, Chevy chassis
2010 Subaru Impreza Sedan
Camped in 45 states, 7 Provinces and 1 Territory