We have been fulltiming in a Pinnacle for almost four years, and the unit has never been back to a dealer for service. There have been a few minor issues which I took care of myself, rather than leaving our home on a dealer's lot to wait for repairs.
If warranty work is required, any Jayco dealer should service it. If they refuse or give you a hassle, call Jayco customer support. If you are traveling when something goes wrong, you have to use the closest dealer anyway.
If you can save thousands of $ by buying online, personally I would do it. The dealer markups on these higher-cost fifthwheels can be very high, compared to the markup on a vehicle like a truck.
You can read about our experiences with the Pinnacle
here.