Yes, mite infestations levels can a good indicator of whether a hive will survive winter or not. However, just because a hive is not treated for mites does not mean it will have high mite numbers or that it will die. A first-year hive started from a package will often survive it's first winter (if something else doesn't kill it). It's in the second season that the classic collapse from varroa usually happens.
Claudia, do you know what the root cause of your colony deaths were in previous years? Without knowing what kills your hives, whether it be mite infestation, starvation, exposure, rodents, queen failure, or too much moisture, a beekeeper can't possibly know what to change in order to improve success. Knowing how to diagnose the likely cause of each hive's death is a valuable skill to have.
I wouldn't move your hive inside the pole barn, but I would consider putting it next to the south facing side of your pole barn or another building exposed to sun. This will provide considerably more shelter than most other locations, and on sunny days you'll get more warming from the sun, allowing the bees more opportunities for cleansing flights and for the cluster to move to stores.
Also, you should describe in more detail exactly how you prepare your hive for winter and be specific about the size and location of all entrances and ventilation as well as your use of insulation. You could very well be over-insulating, providing too much ventilation, or providing too little opportunity for trapped moisture to escape and not realize it.
I like to use foam insulation on the top of my hives to prevent moisture condensation on the inner cover, but if I were to use foam insulation around the hive bodies I would only do so on 3 sides, and put black tar paper on the front. That way you'll reduce heat loss via the insulation but still be able to take advantage of the sun's warmth on sunny days.
Hope this is helpful,