I also have a 17.56.1 and tried both Prestige and Coast to Coast covers at $300 or so each!!! I live on the Coast and find I only get 1 - 2 years max out of them before they become powdery and torn.
My last purchase, about 6 months ago was a cheapie off eBay for about $140 and so far just as good and at half the price!
Would rather buy a cheapie anyday.
PS: get a 18-20ft cover, little bit big but covers gas bottles and all.
Buy a cheap one off eBay. The one on my 20 foot van right now was about $150. It is my third in almost 6 years and there's at least another 18 months in this one.
I think mine came from ozplaza and was a four layer cover. Look for 24-36 months from it.