Chatting to a friend at work who keeps her caravan on a storage site. They have a small solar panel that they sit in the roof light 'cavity' inside the caravan just to keep the battery topped up and say that it works well. I was toying with the idea of doing the same. I think I could fit a 40-50 watt panel in my roof light, so 4 amps max (more likely 2 amps due to light loss through the roof light) then plug the panel via a controller into one of the 12v sockets in the caravan . The only issue I can see is that the caravan 'power' would have to be left on to make the connection between the 12v socket and the battery unless I run a direct connection from the battery to a new socket.