If your van is a daily driver, I would put more emphasis on battery bank capacity than worry about Solar panels on the roof - that's if you tie the alternator in as a charging source.
If you use the alternator, then Solar on the roof is only beneficial when the engine isn't running. So, if you purposely park under trees or in shade while stopped then the panel(s) on the roof will be no help anyway...you're better off using an extended cord with solar suit-case panels that you can position in the sun while you enjoy the benefits of shade. Roof panels will only help when the Van is in the sun, for the majority of the day.
Lot of informative Threads on Suit-Case panels on this Forum - easy enough to use the search function to tap into that info.
Just my two cents.