Bryan,
There are many covers available on the market now.
The most important aspect is to make sure you get a breathable type.
If you get a non breathable one, everything under the cover, both the inside and outside of the van, will be covered in mildew in a very short period of time.
The better type have a soft lining to protect the finish of the van and windows.
Personally we have a Protec, which are not cheap, but are made to measure and are excellent covers.
When fitting it is a very wise precaution to put pipe lagging over every sharp part of the van, such as awning rail ends, water diverters on roofs, lower corners etc, and remove the spike from the status aerial, if you have one fitted.
When tightening the straps. make sure they are tight, but be very careful not to over tighten, or risk ripping the material.