While I mentioned a resort in NE Oklahoma, since I live here I would not winter in my RV (house with fireplace is more comfortable).
When my daughter lived in Biloxi, I traveled there frequently, and found a place enroute where I think I could comfortably winter in a RV at low cost. The area is around Lake Chicot on the Mississippi-Arkansas border, west of Greenville, Mississippi. A lot of the RV parks/resorts are between Lake Village and the bridge. I've not checked out prices, since when my daughter was in Biloxi I would just go stay with here and hang out with the grandkids.
It all depends on how warm you want to be, how much you want to pay. Warmest is in the Florida Keys, which are consistently expensive, a lot of demand for for very little available real estate. It gets cheaper for places less desirable.
However, if cost is really the issue, RV snow birding is probably not the best answer, if you really want to be warm for the winter. Do you know that you can rent an apartment in Cozumel, Belize, or Costa Rica for about $200 a month? (I have friends that do this). Try finding that price for a RV lot in the U.S., anywhere, any time of the year, not to say snowbird country in snowbird season. But you do need airfare and a passport.
While I'm at it, about $250 a month for Changmai in Thailand, $300 a month for Bali. Difference between that and the cost of a RV lot in Florida will buy you a houseful of servants. I've lived in Asia, been there done that.
In south Texas, Victoria Palms, Bentsen Palm Village Resort, Green Gate Grove, Casa de Valle and many more. Friendly folks, great prices, fun in Mexico & much more.
Here are a few Florida parks you might want to research. I've stayed in all of them from a few days to a few months. They are good solid RV parks with plenty of amenities. Prices vary and the further south you go the prices rise.
A bit farther north with fairly warm winters is Hot Springs Arkansas. Catherine's Landing RV Resort with access to Lake Catherine. Back in site $545.99 per month includes electricity. Lots to do in the area. Farther south in Freeport Florida is Live Oak Landing. $674.99 per month includes electricity. 11 miles to the beach. Quiet park, beautiful shade trees. Note: many parks add electric charges to their monthly rate so be sure to compare apple to apples. If you don't care about being near the beach, we also like Williston Crossings in Williston FL. $627 per month plus electric.
I know which one on Florida my cousin goes to, and why (a group of fellow bikers get together for the whole winter) but we've not discussed how much it costs. I'm sure there are other people who avoid this resort because of the bikers congregating there.
One of my own favorites is Cedar Oaks RV Resort in Grove, Oklahoma. Last time I checked it was under $500 a month for a winter stay, electricity not included. There is a winter clientele that looks not for warm, rather a survivable winter, and there a larger group spring and fall, as a stage in a slower migration that follows the seasons.
This year some of my Michigan - Florida friends went back when it got warm early, only to have to deal with late heavy snowfalls in May.