We drove down on April 3rd (a Saturday) and ended up stopping at Exit 94 (outside of Savannah) after about 10 1/2 to 11 hours of driving. It was the week before Easter, so almost all the hotels were totally full. We checked at 8 different ones that had no vacancy, and the Microtel which had coupons for $39 was offering rooms for $99. So, we went a little further down the road to Exit 87 where there were another 6 or so hotels, and stayed at the Ramada Inn for $55. The one thing I can definitely tell you, is don't stay there!! It was the worst hotel I've stayed at in a long time. Very run down. We only stayed there because we were tired of looking and it was one of the only ones that had a pool open for our kids.
I'm sure it won't be so crowded when you go down, though, so Exit 94 might be a good bet for you. There are alot of options there (Best Western, Microtel, Fairfield Inn, Comfort Inn, Country Inn and Suites and many more). If you stop at a McDonalds once you get into South Carolina or Georgia they have Roomsaver.com coupon books (I think you'd probably find them at Visitor's Centers, also). They often have restrictions like 1-2 people, and Sun-Thurs, but sometimes can be useful (of course, not when WE went!)
On the way back we left at 6 am and drove until 5 pm and ended up at Exit 95 in North Carolina (Smithfield) at a Howard Johnson Express, also for $55. Not as bad as the Ramada Inn, but definitely wouldn't recommend it. There was a really nice Super 8 at that same exit (on the other side of the highway), though. I went into the lobby and it looked very clean and new and had an indoor pool and cute breakfast area (of course, when I asked they only had two kings left for $92 each!) They had a coupon for $59 Fri and Sat (I think $49 the rest of the time). But it was near a Bob Evans, Ruby Tuesday, and Texas Steakhouse, as well as a pretty good little mexican restaurant. So if you make it that far, that would be a good place to stop.