Caravan covers

Hi all,

We need a cover for our potop, Avan ErinII approx 17' 3" long or 5200mm, 2370mm wide and 2300mm high with potop up.

I've seen the ones on Ebay, but they are all different prices, from $99 up to $350! Has anyone one got experience with covers, should they be totally waterproof or just the roof waterproof with the sides 'breathable', is best to have a zipper or access on the side/s? Any good makes/suppliers and whats a reasonable price.

We will be going to the Perth Caravan/Camping show this weekend, but would like some knowledgable background if possible.

Thanks everyone.

Ian and Deborah