Your cheapo atlas is right; US98 is a 2-lane road that winds along the coast through tiny fishing villages before going through the wildlife refuges of the Big Bend region. The Port St Joe Scallop Festival (a VERY cool little fair which was, oddly enough, utterly bereft of scallops) is held at the end of July. There's a very nice dockside hotel with an attached seafood restaurant right on the water in Apalachicola. Perry is the first significant town past Apalachicola, so gas up before heading into the Big Bend.
You can stay on US98 the whole way to Perry; in fact, you can stick with it all the way to Homosassa (fabulous park at Homosassa Springs - WELL worth the price of admission) but at that point you have to start looking for US19. Now US19 is a beautiful road for riding north of Homosassa - four-lane divided highway which is largely abandoned since I-75 came through in the 70s. But below Homosassa it turns into traffic purgatory. The Sunshine Skyway Bridge is very cool and well worth seeing, but you should pick up I-75 once you get to Pinellas county.
Mobile to my house in the area north of Tampa takes about 10 hours on this beautiful route, though you can shave three hours off of that by pounding along the interstate.
