I have always had a cornerstone warranty on any coach that I owned. I will tell you from experience, that the rear end went out on my 06 Knight and I am thankful for my cornerstone warranty, it was a total of 7,000.00 to fix the rear end and my out of pocket expense was around 1500.00.
I have a cornerstone warranty on my Tuscany that covers everything in and out, no not tires, it cost me a little over 6,000 but it was for 10 years being that it was a new coach. 100.00 deductible or 50.00 deductible (where I purchased it).
If you want an extended warrant, just from experience, I would recommend cornerstone and not through a dealer,