Don't remember the brand off-hand, but we use solar powered trickle chargers for the cars that will be sitting on the lot for a while at the dealership I work at. They work extremely well, never have issues starting up cars. Even if its old used inventory that hasn't been started or driven for months.
Now that I think about it, the automotive school I went to used similar setups for the cars they had for us to learn on.
Something to look into, I suppose. That is, if you'll be able to leave at least part of the windshield/back window exposed to the sun.