Take the money the dealer wants for the extended warranty and put it in the bank/CD whatever. It won't generate much interest but it will be there if you ever need it and most folks don't. Given the litany of complaints regarding getting service from dealers with the original warranty know that it will only be far worse dealing with an extended policy, usually issued by a third party.
The first year or two should be covered by the manufacturer on new coaches, on a used coach your best bet is a thorough PDI and test drive. Once it is yours maintenance is your best friend. Extended warranties usually benefit the party selling it and no one else.