I'm curious. How many other horses are left and who do they belong to? Typically when a barn decides to close down, everyone is given a date that they have to be out by. Did they just expect you to take care of these horses indefinitely? At the very least, if the other owners know the barn is closing, they should be getting those horses out of there. I would give a notice now, and move your horse. Stick out the feeding as long as you think is necessary, but definitely move your horse. If they do sell the property, the new owners are probably not going to want a bunch of horses there, and if they give you notice to move, and your spot at this new barn is gone, you'll be struggling to find something fast. It isn't worth the risk.