My son rode his 09 Fatboy from Central Wi out to Cody Wyoming and back in June. He has HD backrest/luggage rack, HD saddle bags and a HD strap on tour pack. The group put on over 3,000 miles in 2 weeks. He lost his rear master cylinder from the bike in front of him kicking up a rock and shattering it in the Mountains, so he had to ride all the way back without rear brakes. I had the dealer at Rapid City check for a master cylinder in SD and MN and the only place he found 1 was in Wi (our local dealer where he bought the bike new). When he got back our dealer fixed it for him UNDER WARRANTY! He said the stock seat got pretty tiring after 400 miles or so and his wrist got really sore. He is 21 and had no real problems, I'm quite a bit older and we only rode as far as Rapid City, but we were on baggers.
We took a nice ride last summer up to Door County in Wi and put about 600 miles on in 2 days, so a Fatboy can be a great long haul bike with the right mods. Good luck and have fun!