I don't know about "many snowbirds" but the Mississippi Gulf Coast, southern Mississippi, most of Louisiana, the Texas Coastal Plain (which reaches to San Antonio), and Florida's "Nature Coast" fit my idea of what is comfortable in winter. It is going to be quite warm sometimes, quite cool other times, and Louisiana to Florida, gray and/or wet a lot of the time. But not freezing long enough to be a problem for my RV.
I thnk a lot of snowbirds, particularly RV snowbirds, are looking for something warmer, bask in the sun weather. My cousin leaves NE Michigan at the end of September and doesn't stop until south of Fort Myers. On the other hand, my recently deceased brother spent 10 months a year in Northern Florida, two in South Central Michigan, houses both ends. Snowbirds with houses might be more tolerant of cooler days. A lot of my family likes the west coast counties north of Tampa as winter (or primary) home locations.
I lived two years in central Florida, found it too warm, even in winter. But that's a personal preference, I like a little winter weather, so I stay where I get a couple months of what I would call winter, having grown up in Michigan where I got twice as much of it. It is summer that I move for, most often.
Tom Test
Itasca Spirit 29B