Like Peter T we have a largish solar cell, in our case a free standing one of 85 watts of the crystalline type.
Through most of the year we can be completely self sufficient as the electrical demands other than winter are not that great and the input is generally well up.
However during the winter it becomes much more of a lottery, the demands are up and the yields can be pitifully low. If we have some nice bright sunny winter days we do get exceptionally high yields, most likely as the air is so clean. However on a dull cloudy damp day the cells yield is but a few percent of its rating and if we get a week of these we run out of power.
If your an at all serious camper as opposed to a three day weekender then my advise is to pitch things at more than 40 watts, ideally somewhere near Peter or ourselves at 80/85, then for most of the season you would need to be profligate with power use to be in trouble. If your going to push into the late October to early March use then a free standing large panel is what I suggest you need. Free standing because you are going to have to stand it up at about 50 degrees to catch what little is available and mounting horizontal, as on a van roof will greatly inhibit its yield. Mounting flat on the roof carries very little penalty performance wise other than in those "winter" months, and has some practical advantages.
As I said ours is a free standing one, and that's because of when how we need it to work. To live with such a large panel I made a cassette for it under the back of the van, screwed up into the floor; that works very well.