I have a couple thoughts about this situation. The first one being that as a boarder I would be extremely upset if the barn owner left town and didn't say anything to the boarders. That is very unprofessional. Second, until the barn owner returns I would try to help the girl that is working at the farm to get through the week. This will ensure that the horses are being fed, watered and taken care of. She might need help and it is hard to say what conversation took place between her and the barn owner before she left. If the other boarders are unhappy and causing chaos then they are making the situation worse for everyone. I would try to talk to a few of them and band together to help this girl with the chores and when the barn owner returns you can address your concerns with her directly. See what her response is and go from there. If you do not feel comfortable with her answer then you have the option to move your horse.
You had mentioned that you have been happy with no issues for the last two years. If you want to move your horse that is your decision but I would first find out if something else is going on before you make that decision. There might be more to the story than any of the boarders know. It doesn't make it right that she left without telling anyone but I would do some investigating. I know that if I had left town unexpectedly without telling my boarders then I am sure they would be very worried that something terrible had happened because they know how I normally run our barn. It would be very out of character for me to do that.
Fear of giving a thirty day notice is a common fear that many boarders have had. I hear stories all the time about a person giving a thirty day notice and all of a sudden their horse is not being taken care of at all. I don't understand why a barn owner would ever dream of doing this but it happens. To me it is completely unacceptable behavior and very unprofessional.
If you are looking for a new place to move then I would encourage you to do what your contract states and give the thirty day notice. You might need to watch what is going on a bit closer and if things start to go down hill, then I would forget about the money and take a loss and move the horse sooner. Trying to save a couple hundred dollars is not worth it if the care of your horse is compromised.
I have had my share of extremely bad boarding experiences growing up and you never forget those things. As you look for a new place to board your horse these situations that you are going through will make you much more aware of what to look for in a boarding barn.
Thank you for sharing your situation with us so that we all can learn from it.