It sounds as if you have a lovely set-up. I would suggest leaving the greenhouse empty when you go south and refilling it when you return. If there's no-one to enjoy it from Christmas to June, why bother heating it etc? Save the money and use it to buy new plants on your return. If you are bringing in garden plants you wil already have had the pleasure from them. Hardy perennials will overwinter just fine without heat and you can put them out on your return. In June, fill the greenhouse with heat-loving easy and colorful annuals such as petunias, calibrachoa, verbenas, begonias, cannas, New Zealand flax, dahlias, caladiums, various daisies etc. These will not require any heat after the summer has heated up and you should give them full ventilation at that time so it doesn't get too hot.