How long and cold before line freezing problems?

I also think your RV should be fine in the weather you've been having. It's (comparatively) well insulated, a little bit like a picnic cooler just operating in the opposite direction, keeping the cold out and the warm in.

I'd be more worried about the spigot the hose is attached to, and even there I don't think the chances of damage are great.

And, as others have suggested, you don't want to put off winterizing too long.