Extended warranties are like any other insurance policy, in other words, a gamble. If you have a major failure, they can pay for themselves very quickly, if not, they may actually cost you money over their life. Do not buy one from an RV dealer, go to an independent (like Warranty Experts as Shayne suggests, Dave sold us our warranty) and shop prices yourself.
Be sure to get an exclusionary policy, one that lists only those items that are NOT covered.