What a co-incidence, last week I persuaded my Mum that solar panels were now cheap enough to pay for themselves in a realistic amount of time, so we bought 4*190W panels which will go on the shed. I've already got two suitable inverters, the batteries and cable (can get that from uni too), so it's going to be fairly cheap to do. I've now started designing an MPPT controller as they don't look that difficult, just a buck converter...famous last words perhaps!

Anyway, like m_c I'm not sure what you're trying to achieve here? Taking 12V dc from the van (battery, alternator?) through the controller into what... some batteries? If so why? Oh are you charging the van battery with solar to offset the power drawn from the engine via the alternator?