We have a Kyocera 120 and a K60 (space limitations) on the roof of our ATRV with a RV Power Products Model 2000E Charger Controller/Booster. During our entire trip to the Arctic Circle last August/Sep I never had to use the genset to re-charge the five (yes, [I]five[I]) deep-cycle batteries on the trailer.
This is overkill, but I like to err on the positive side. When we set up our solar system on the house in Baja six years ago we determined what our 'needs' would be, figured out the number of pv panels/batteries and doubled it.
Mario gives excellent advice above. I have that book he mentions and I do recommend reviewing or purchasing a copy.
If you're wanting to only maintain your vehicle battery over several (many?) days of non-engine charging, a small pv panel/charge controller is all that's required. If, however, you have a 12v fridge, a blender for you margueritas, a couple of exterior 12v lights (flourescent or LED of course) for the campsite, then you'll have to increase your pv wattage. That book is an excellent guide.
"I'm ahead of the game and can afford to take chances." --Theodore Roosevelt
'11 Mini Cooper Clubman, '04 Dodge Dakota 4X4