You might want to talk with that dealer again, and get a copy of the 2008 Accessories and Parts catalog. Several of the Harley saddlebags, leather, synthetic, and hard, will fit your Fat Bob.
The only hang-ups listed in the catalog are an incompatability with rear axle covers and, for the hard bags, with detachable sideplates. You don't need to relocate the turn signals.
Or is he saying he can't secure them even though they are listed?
Good luck, and let me know how you like your Fat Bob. My only buyers remorse with my '07 Super Glide came when I saw my first Fat Bob.