I've got an extended 2001 SMB and I just use one of the standard van covers, and loop a line over the top to hold it down in wind. Pretty easy to just lift up the skirt so to speak for access. I wouldn't want zippers gnawing away at my paint.
I had a van cover and now use a Home Depot tarp. I find it easer to put on and take off. It is held on with bungee cords. Cheap and works great.
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Current van: 2002 Ford E350 extended body camper with Colorado Camper Van pop top and Agile Offroad 4WD conversion.