Is 31 degrees the forecast for the overnight low, or the daily high?
If it's an overnight low AND the forecast is accurate, I would not worry about it at all myself, particularly if your trailer has enclosed plumbing. It takes more than a few hours at barely subfreezing ambient temperatures to freeze the water lines solid and cause trouble, and the RV is insulated enough that the interior temperature probably wouldn't get below freezing anyhow.
If it's a day or so that's below freezing rather than a couple hours overnight, you ought to winterize. There isn't really any such thing as "half winterizing" for half winters; if it's cold enough for the plumbing to freeze, it's time to winterize even if it's just barely there.