solar charging battery

My caravan is in storage about 30 miles from my home, in the winter I have had alerts from my tracking company to notify me of a low battery, although the battery was fully charged when I left the van, resuting in having to collect battery, recharge and return it to van. I would appreciate any advice members can give me regarding trickle charging the battery with a solar power charger.i.e. do they work ?  recommend a model or manufacturer. Because the van is covered during the winter I would obviously have to fix it on top of the cover by some means which shouldn't be too difficult.