Is it possible for the septic tank or main pipe line to freeze in the winter months?

Yes, depending on the depth of the pipe and depth of frost. Snow can insulate the soil, so there’s less chance of freezing if there is snow on the ground. Compacted snow will not insulate as well as un-compacted snow. Driving any equipment over the system will compact the snow and send the frost down deeper into the soil. If your tank is filled to capacity effluent could back up into your pipes and burst if the contents end up freezing.