How to make a mob that moves around one specific spot

I'm wondering how to make an entity that still has a normal wandering function, but also a camping function where it will spawn at my custom structure by default and then stay there. I want it to wander around a bit, but return when it gets out of a 20 block radius. Right now I am using an entity that is invisible invulnerable and is under the center of the campsite. And when it checks for this entity if it is not in a 20 block radius of my camping entity then it will again check for it but this time in a 35 block radius and move towards it. But it is crashing my game whenever I spawn a boo cow with the custom nbt that decides whether its camping or not.