The dealer is right, factory requirement is 15K miles or 12 months service intervals, whichever occurs first, for the 5-liter V8 engine. Diesels have different requirements, not sure about those. Many opinions, here and on other forums, on whether that's too long or not. I replace my oil every 7500 miles as well, as it is my car and I plan on squeezing at least 150-200K miles out of it. I do put about 30-35K miles a year on it, so an oil change is a fairly frequent process for me... Finding the right oil for this car is the hardest part, the process itself is very easy, with the right tools you can do it in your garage in 30 minutes or so and you will barely get your fingertips dirty.
If you are leasing the vehicle, don't sweat it and stick to the dealer's recommended schedule. If you own it it's your call, but make sure you use compatible products (synthetic oil, filter) if you will be performing more frequent oil changes than the dealer's recommended, for warranty purposes.