I've been thinking about how to do this with the Sprinter we have on order. Same situation - TH's and ski lots in the winter here in Colorado. As well as the red-eye drives to jackson.
I've ruled out cracking windows and using a roof vent. They both will let in spindrift if you have a fantastic fan. Plus, you're airflow will be a function of wind speed, so windy locations could mean lots of heavy drafts coming in.
I thought about venting the D2 to pull exterior air, but then you're dealing with dust or adding filters when it's summer (but cold). I've settled on 2 different ideas
1 - Espar maks an HVAC controller that turns on the factory fan when the car is shutoff. This is pulling air through the HVAC filter (no issues with dust in the summer), draws a small amount of power, and fan can be setup to run really low to trickle in the fresh air. I saw the option listed on Lubrication Specialist's website. I'm sure there are other ways to wire a switch and power to the HVAC fan, though.
2 - Maxxair fan. This is the alternative to a fantastic fan. Same hole cutout in roof, but the maxxair takes up a few more inches of longitudinal space on the roof. The air intake is always pointed down, so not much issue with spindrift. It also has ten speeds with the lowest pulling 0.1 amps. Because of the hood design, you can power it rain or shine, driving or stationary. I'm thinking that cracking the hood a tad, putting it on 1 or 2 for a speed setting, leaving the windows shut, the fan will then draw air in from the HVAC vents as long as that system is left in fresh air mode. It'll be just enough to introduce a steady stream of fresh air without dealing with spindrift coming in through the windows or fan.
Not sure how the webasto's are, but the espar units tend to get carbon'd up at high elevations (even with the high-altitude kits). I've spoken with Thermo King here in Denver, and they suggest finding ways to get the Espar to run at high power to keep that carbon build up to a minimum. Or, run the thing on high once back at Denver elevations to clean it out. So, having a bit of air coming into the van should allow the furnace to run at a higher setting to keep things clean.