It looks like one possible route that might be flatter would be to go through
Texas, but make sure you double check everything if you're planning a road trip.
Elevation data comes from different sources and may not reflect true
driving conditions, so talk to other RVers if you're concerned about
steep grades. If you're planning to drive in the winter and you want to avoid snow, check the
road conditions from Ohio to Florida. You can also view a 3-D map of
Ohio elevation or
Florida elevation.
If you really want to avoid mountains, the best route would probably be to go down I-71 to I-65 through Nashville. That gets you around the Smoky Mountains, then you can cut back East to Florida. It does add some extra time, but it's not a huge amount and you avoid I-77 which can be very hilly.
Another option is to take I-75 through Knoxville. That's not as flat as I-65 but it's a little less out of the way. It depends also on where you are starting in Ohio and your final destination in Florida. Remember that Atlanta can have really bad traffic, so you would need to time your driving to avoid getting stuck there.
Also make sure you check the weather conditions for any route you choose!