I have about an inch on roof of 23B. It is also under a tree that is dripping ice and leaves. I had the trailer level but have tilted it to lean forward so ice will run down front when it melts. It...
While antifreeze is a good idea there's room for ice to expand in the P-traps so you're probably OK. Water tends to vibrate out of them on the road and they may be dry anyway. (Twist locking the sink stoppers closed on the road keeps the stink from the waste water tank from entering the trailer.)