The big questions for panel sizing is how you spend power at night (when the engine is off), and how many nights you will stay in one place without driving the van for more touring. You could always use the van as an inefficient generator if you get into battery trouble - but most vans do not like to be idled for more than a few minutes without motion. Actual driving, in comparison to idling, creates airflow around the exhaust components - and mere idling can overheat those things (manifolds, mufflers, catalytic converters, and so on).
I have a Trailer RV which charges at about 450 watts from the TV, but we tend to stay in one place for multiple days when we reach a gorgeous destination. So my Solar has to do all the recharging - except in "emergency" situations when several cloudy and rainy days have happened in a row. My 4Runner is pretty good about extended idling, and Toyota has no particular warning to avoid it. (Although, per above, your van might NOT like doing that). I have 500W of panels overhead, flat on the roof with no angle into the sun. In relatively sunny days in late summer, we're usually fully recharged (from Solar alone) before 2 PM daylight time.
So I'm a bit "over-paneled" for my actual electrical use. Your 400W pan sounds pretty good to me. If you don't go for drives every single day, or you extend your travels into fall or early spring, then I "feel" that only 200W of panels would be marginal at best, and maybe inadequate.