It may not be what you want, but if your heart is set on Zihuatanejo for the winter (and we SO get it) you may have to take another approach and be flexible. Instead of trying to find a solid three month chunk of time, just take what you can get. Show up, take what is available. If you have to leave for a while, check at Real de las Palmas. Try Playa Linda, hopefully get another month or whatever, have to leave for a bit and return for more. Acapulco isn't far away, an easy drive. We always enjoyed it too!
I think the only way you are going to get your foot in the door at either RV park here in Zihuatanejo is to be flexible, show up and get established so that if this is your paradise, you can get your 3 months in the future.