I never have.... but always consider it, depends how handy you are to tackle a problem and if you want to tackle it yourself or pay someone. Also depends on your access to a local RV repair shop.
Here is a good link and pessman discusses Extended "Warranties "
Mileage..... in our area the average is about 4K year. Of course some do alot more.
Traveling..... we start in March and stop in November. Many weekend trips, 3 or 4 days. And long trips in the summer.