How about the Gentle Repose spell?
When I do things like that, I like to have my zombies pretend to be corpses for whom I wanted to give a proper funeral, if someone asks at the edge of town, and at the edge, I bury them and set up some sort of marker - I've had a portable grave marker with this character. When you leave town, dig up the zombies. Of course, Gentle Repose probably won't work in tandem with this, but casting Gentle Repose will keep them from rotting, as long as you continue to cast it.