It's a crapshoot there as in much of the south. Averages are mild, but hot or cold snaps are always possible. I was in New Orleans a few years back a week for the Krewe de Viewx Parade and temps were near freezing, Other times in winter can be very warm.
Google Natchez Ms. and go to the wikipedia listing. It will show you average hi's, lows, and the records for each month.
In general on your rv, if you run the furnace occasionally on the coolest nights, your tanks will normally be fine, your water supply line could get a wrap and tape job if you hit a cold snap.
Happy motorin