Solar Battery Charger for Enclosed Trailer

Anyone using a solar panel charger / battery maintainer for their enclosed trailer? I was thinking a 5-watt panel would be enough to keep the battery fairly well maintained. I just use if for lighting when the trailer is not hooked up to the truck. I may slap a stereo in there at some point. I have two 15 or 18-watt panels on my camper that seem to do the trick all summer. So I figured a 15-watt panel would be over kill. Any thoughts?