"Best weather" is subjective.
You later say "around 70 most of the time" and the place in the continental U.S. that comes closest is the San Diego area (ocean side) where the winter temperatures tend to be in 50-70 F range without big daily temperature swings. However, winter is the rainy season, and the reason for not so much variation night to day is the temperature stability of the Pacific Ocean, carrying over to shore as moist air masses. Ocean temperature stability is also the reason the most popular wintering places are along the shorelines of South Florida, and just as with San Diego, this popularity and demand for housing makes "rent" quite expensive.
Once you get away from the ocean, temperatures are not "around" any particular figure "most of the time" because solar heating of the surface and radiation of that heat when dark or cloudy means average temperatures swing over a wider range day to night and day to day.
Desert climates have the biggest daily swings, and more variation as weather systems come through. Yuma, for example, enjoys daily highs in the 70-80 range November through March, but average daily lows are in the 40-50 range. Tucson's average daily highs are about 10F lower each of the winter months, the average daily lows 5 F lower than Yuma. In both places, short term extremes can go another 10 F higher, another 20 F lower than daily average lows and highs.
Rio Grande Valley, you can expect average daily highs around 60-70F during the winter months, with daily lows as much as 20F lower. But the extremes here can be scary, December and January highs have been below as 40F, as high as the mid-90s. Extreme daily lows have been in the 20s, but have also been as high as 70F.
The sort of climate I think you seek is most often found in tropical Oceana, where the latitude puts the climate in a warm zone, but the ocean keeps it from getting too hot from solar heating during the day. Moderation from ocean temperature stability also means that extreme highs and lows are fairly close to average highs and lows. Hawaii comes pretty close to "best", with winter month lows 65-70 at night in Honolulu, highs not much over 80 F during winter. Hawaii is far enough from the Equator that it does have measurable seasons, but they are subtle. But Hawaii is not a place you can drive your motorhome.
Your cost concerns are probably going to keep you from finding the "best" weather, so you might have to settle for the warmest weather that you can afford. I suggest the less heavily populated areas of the desert Southwest, and to be more consistently warm, this means somewhat south of Phoenix or southwestern corner of Arizona, southeastern corner of California.
I've lived in Central Florida, have wintered on the Gulf Coasts of Florida, Mississippi and Texas, and while often pleasant in winter months, the places are not frost-free and are subject to wide variations in local conditions as the continental air masses sweep through on a almost weekly schedule.
Tom Test
Itasca Spirit 29B
