If you use the search function, there have been dozens of threads on this (they are not extended "warranties," they are insurance policies). Grand Design does not offer one.
My wife found a very good description of extended service plans a while back (as stated, they are not warranties, but rather insurance policies with lots of fine print). They are another form of legalized gambling. The company is betting that nothing breaks that would cause them to pay out more than you paid for the policy; you're betting it will. As in all forms of legalized gambling, the house usually wins. Otherwise, these companies wouldn't be in business or make a profit.
Dave Ramsey and Clark Howard have commented on these plans many times, too. The standard seems to be that 80% of extended service plan revenue goes toward commissions and marketing. Only 20% goes toward paying for actual repairs, which means most people are getting a really bad deal. We prefer to set money aside regularly (earning interest) and pay for repairs as they come up.
Rob