dadoubleren, I have a 2007 Fatboy and also got only 5K miles on my OEM rear tire before changing. Same as you, I was a little miffed about what I thought was pretty low mileage so I e-mailed Dunlop requesting some comments and ask if they planned to make changes to increase mileage in the future.
Their reply in a few days stated that the life expectancy of the 200mm tires is between 5K and 8K depending on the individuals riding stile and tire pressure maintenance.
I live in Oregon and most all my riding is on asphalt roads (rather course) and very little if any on concrete freeways. I think this has a lot to do with my low mileage. Basically the tire compound is a little softer than the smaller tires such as the touring bikes have.