Mice, snakes and other vermin are going to be looking for a warm place to stay for the winter, whether the Monty is inside or out. IMHO, you are wasting you time putting out Decon or any other poison in a barn. Growing up, we always had cats and black rat snakes around the barn and had minimal mice problems. You would be shocked how many mice 2 cats can eat.
Maintenance is very minimal with barn cats. Just get a small feed tub at the local farm store and buy a 50lb bag of cat food, keep it in a plastic or metal barrel with a tight fitting lid and once per every 3 to 4 days fill the tub and you'll have some happy barn cats. Once the mice population starts to dwindle you can feed them more often. If you don't have livestock you'll have to figure out a heated water setup also, but that is easy enough done too from the farm store.
I envy you as this is bringing back many memories of my childhood, just writing this.