Depends when in the year you caravan. In principle, I totally agree with Peter T. However, I use a smaller 30W panel. I originally bought it with the intention of buying another (must be identical) one later, but didn't bother as it produced sufficient power for my purposes. In the winter, I carry an 85AH battery in addition to my 110AH elecsol that I carry on the van. The smaller battery powers my tv and as I tell the kids - when it's gone it's gone!! Panels don't produce much power in the winter. In the summer it's back to the elecsol and the panel - mounted on velcro strips on the roof.
Also, you should make sure that you get the right panel. You get what you pay for!! The better panels will start producing power at lower light levels than the cheaper glass types. My panel was purchased from "windandsun.co.uk" Can't remeber the price. It consists of bright blue crystals bonded on to an aluminium sheet. The whole thing is then coated in resin so that it is semi-flexible. It will not break like the glass types. On a good day it will fully charge my battery.