Route from Illinois to Disney World

I avoid that Chattanooga drop off like the plague. I think it's actually a 7% grade. Did it ONCE with a MH towing, never went that way again.

Need to know from WHERE in Illinois you are leaving from.

I made that trip for years and I have found some 'very' pleasant routes to take with an RV. All of my routes ended up on I65 to Birmingham and than east on 20 to Atlanta and then it's I75 all the way down.

Yes there is a flatter route from Illinois. Basically you need to get over to I-65 and depending upon where in Illinois you live that will taken you eastward or at least southeastward. Follow I-65 all the way south to Montgomery, Alabama. From there catch US231 down to I-10. You can take I-10 over to I-75 or you can follow US19 down along Florida's west side and catch another route over to I-75. You'll need to hook up to I-4 in Florida if you want to stay on the Interstate.

There are some hills in Tennessee and some rolling land in Alabama, but they are certainly not mountains.

And welcome to RV.net!

Southwest Airlines?
But seriously...the `mountains' in the East are just hills that one can drive over in a half hour or so. Nothing to worry about. A good resource is Mountain Directory East.
The only real `mountain' we experience between Chicago and Orlando is on Route 24 just west of Chattanooga (MM 129 - 134) where there is a 5% grade.
My notes state that we were doing 30 mph going uphill seemingly forever but more like 5 miles, then in 3rd gear (out of 6 gears) going downhill (in towhaul mode); towing our RV.
Stay out of cruise control or you will use up fuel badly.