Why not just take them with you when you leave? Put them in your basement or somewhere that stays above freezing and don't worry about them. That small a panel probably won't keep up with just your phantom drains. Putting a larger solar panel on without some sort of regulating charge device is asking for trouble. If you really want to keep them charged, you'll need about a 50w panel (these are quite small, and a single 100w is still small) and a solar charger (a Morningstar TM-45 will cost well under $200 and leaves a LOT of room for expansion). Won't have to worry about boiling your batteries!
2010 Montana 3665RE, wet bolts, etc.
2006 Ford F350 Diesel Dually, air bags, Softopper, Aeroshild, etc.