Hi and welcome to the forum.
If you've looked through old threads you've learned that the membership is divided on this topic... but I'd guess it's tilted in favor of "self insuring" instead of buying an extended warranty.
We were happy with the six year policy we bought from Easy Care but I don't recall hearing anything about Cornerstone... although I did a search of the forum on that vendor and got a lot of hits so you might try that.
Best of luck.
Rick, Nancy, Peanut & Lola our Westie Dogs & Bailey the Sheltie.
2007 Itasca Ellipse 40FD