Explain what you mean by "affordable" and "warm" and we can probably be more specific. You might also clear up what you mean by "campground" because costs depend on your expectations.
You will find places you can park, where winter weather is mild to warm, for no fee or a modest (few hundred dollars) seasonal fee, on public lands in the Southwest, particularly in desert areas of southeast California and southwest Arizona. These will not have utilities, you will be providing for yourself, generating what power you use, hauling water in and hauling waste out, or paying someone to do that for you. Unless your needs can be met by solar power, you will also be buying and hauling in fuel to generate power.
Next up on the price scale will be public campgrounds with hookups for utilities, and modest RV parks, which you will find open all winter from Oklahoma-Arkansas-Tennessee and south; I'm familiar mostly with places in East Texas, Louisiana, and the Mississippi Delta country in SE Arkansas and western Mississippi. Prices depend on demand and facilities, demand depends on location, facilities and temperatures. At the northern end of this range you might find full service RV parks available for $300-400 a month, but you might be paying for electricity, cable, Internet connections on top of that.
The further south you go, the more consistently warm it will be, and the higher the monthly rents. But even going all the way to the Gulf Coast, you will have nice days mixed with days (particularly nights) that go below freezing. Not consistently very cold like Dakotas, Minnesota, Iowa, Kansas and most of the western Plains (even into West Texas and New Mexico), rather sometimes moderately warm, sometimes moderately cold.
For consistently warm, almost always above freezing, most of the time spring-like to MidWesterners, you have to go to South Florida, the southern tip of Texas, southwest Arizona and Southern California. South Florida and Southern California have RV parks (not campgrounds) that cater to people who want to winter in a warm place, but most are resort-like (because it is expensive generally to live there) and may be $600 to thousands a month. The RV park bargains are in a few Arizona locations and in South Texas; no specific recommendations because I go the other way in the winter, toward the Gulf Coast and the parts of Florida where it can still get cold.
If you want "never below 70F" you will not find that in the Continental United States. I've been in Key West when the temperature dropped below 50 F, it can get below freezing at night in the southwestern deserts, it has snowed in Corpus Christi and El Paso, and within the past two years it has dropped below 50 F in Southern California, even right on the coast.
