The general rule of thumb I've always heard is that it takes around 24 hours at 32 degrees for water to freeze in a pipe. When it gets into the 20s, it can happen in 6 hours or less and in the single digits/teens, in 3 hours or less.
If you don't have plans to use your trailer, definitely wait until April to de-winterize. If you do plan to use it, make sure you're full up on propane, de-winterize, and keep an eye on the projected overnight temps. If it looks risky, plan on running your furnace until it warms up. Just be aware that if you run out of propane, you could have an expensive disaster on your hands.