Joy wrote;
>>Is all of this weather damage worse than what happens when living in the van
>>on the road for two and a half years?

I think so.
When the van is being used it is exposed to light and weather from all directions, when the van is stored out doors most of the light and weather will hit one side (sun from the south and rain etc. from prevailing winds).  Dirt and leaves will tend to lay on the van instead of blowing away while driving.  I'm not sure (ask an engineer) but I think normal vibration and movement from driving helps keep the rubber and plastic parts "alive".  When you are using your van you will tend to wash and wax it more, clean the counters and cabinets, dust the dash etc.  I think all the attention of regular use will keep the van fresh.  
Lots of use may also have its downside, more mechanical wear and more chance of bumps, dents, and tears.
Use the van as much as you can and if you can not use it keep it somewhere out of the sun and weather.  Before you store the van make sure it is clean and waxed.
George
'85 Westfalia