There is no "freeze line" along your planned route, in the sense of temperatures never going below freezing. You might assume New Orleans to be less than 5-10% chance of frost, but that increases as you move inland again along I-10. I've been caught in ice storms in Houston, and sub-freezing whether is not unusual at San Antonio, and occurs more often as you move west through El Paso and western New Mexico.
But you are just playing the odds. On many nights it drops below freezing, the days may be above freezing. Or maybe not. How long does it have to be below freezing to cause a problem with YOUR RV plumbing?
I've had pretty good luck with dewinterizing at the I-30, I-20, I-10 corridor, for San Antonio and east, December and January, because warm Gulf air pushes in that far. Maybe 40s-50s during the day and 20s at night, not enough time for tanks and lines to freeze if the furnace runs at night.
West of San Antonio, I'm not so sure, elevation is rising, continental air masses out of the Artic push away the warmer Gulf air. February? On the high plains and hogh desert, that's often much colder than earlier in the winter, until spring patterns emerge again.
Always above freezing, no way. I-10 corridor is not frost free. Warm enough for dewinterized travel, quite likely, if you are prepared to generate enough heat to take care of things during subfreezing peroids.
