I-75 South with a by-pass of the hills in Tenesee

As others have said, work your way over toward the Mississippi River and long grades will be eliminated (you do get some short up and down hilly areas though)

Are you drastically underpowered or something? The I-75 route isn't extreme and if you have reason to be scared about that, I would be scared to make a 1500 mile trip.

It's a lot of extra miles just to avoid a short section of roadway.

I would hardly call those mountains, large hills sure. I've driven that several times pulling a trailer and it wasn't a problem at all. Just take your time and fall in behind the semi trucks going up if you want. I've driven I-77 once going north in a snowstorm pulling a trailer though and it wasn't fun. A lot of hills and some tunnels.

The only long grade is at Jellico Tennessee right at the KY-TN border. Even then it is three lanes with the far right being a truck lane that 45 MPH is common. I would not be afraid of that area, just get in the right lane and drive at a speed that you are comfortable with, you won't be alone.

If you go the US 98 route I advise you not to get off of I-10 until after you are east of the Destin area. Terrible traffic around there.

If you're taking I-75 through Ohio just split off on I-71 southwesterly from the Cincinnati area toward Louisville, KY. Then take I-65 south to Nashville where you can take I-24 back toward Atlanta and I-75.

Another option, the route we take, is to follow I-65 south from Nashville to Montgomery, AL and then US231 down to I-10. Depending upon where you're heading in Florida you can head further east to I-10 or take US19 south along the Gulf Coast of Florida. When taking US19 you can always hook back up with I-75 by catching US98 through Brooksville, FL. It's all 4-lane (US231 and US19), but US98 is 2-lane without much traffic. This is a pretty flat route to Florida.

We've driven I-75 up to the Canadian border and I don't recall any steep grades. It is hilly around Chattanooga but it never crosses any mountains. I have to admit to being insensitive to grades however. We went up Moki Dugway twice this last summer, didn't bother me at all tho the wife was white knuckled. But except for the hills around Chattanooga I-75 is mostly flatish. There are some hills in Georgia but they don't amount to much.

I-55 to Montgomery then southeast to fla panhandle.

This past October we went as far south as Knoxville and 75 was nothing to make you nervous.