Camping in Spain in the winter can be nice especially with the weather between the hours of eleven in the morning and four in the afternoon. You can normally rely on clear blue skies and temperatures around the 22 to 26. It's certainly a tee shirt, shorts and sandals dress code. But, once the sun starts to go down, and temperatures start falling down to a possible ten degrees, it's on with the fleece, long trousers and trainers plus heating on. You can, of course, be very unlucky and, when it rarely rains, it can be horrible, they have a thing called the Gotta Fria which you do not want to experience.
Your right about “wild campers” they are just not welcome. Everybody in Spain considers them to be parasites (like the “travelling community” who do not bring anything to the local economy, dumping their waste (including toilet contents!) and blighting the landscape. Camp-sites can be found and there you will be safe and welcome. I would not recommend wild camping in Spain.
Some camp-sites, in particular, are very, very popular with campers pre-booking and then staying for long periods when they will get a much better daily rate. Yes, you will be surrounded by snow birds from across Northern Europe (with Brits as well) who enjoy the sun and socialising with their new found pensioner “age palls”. It is a harmless way to live and for many, it is a far better way to spend their winters, as opposed to just not getting out during the depressing, sun-less winter months back home.
It is, as they say, “a small world”. The above third photo provided last Monday by bvddobb (Bart & Manga who I do not know) is of a front line camp-site in Altea, on the Costa Blanca, which I do know very well as it is nearly next door to my Spanish home. You can see to the right of the photo a low block of flats of and I am lucky enough to own one of those flats! Will be back in Altea mid-February with my VW Ocean having sorted out a medical issue. Good old NHS and yes I do still UK tax. The camp-site in the photo only opens late September to early April and holds about 100 campers (no caravans or tents) and in that time it is nearly always FULL.
In short, go for it and enjoy the Spanish winter sun, I did - but do plan to stay in proper camp-sites.