They have much bigger ones then that, that 5watt was the second biggest I saw. The smaller one was a 1.8watt I dont know how big your trailer is, you could use the entire size of your trailer for a solar panel. Of course the bigger panel you go the more dollars it will cost. The 5w is around thirty bucks. But I have seen some at Harbor frieght store alot cheaper.
The one from the photo is from northern tool.