We live in Orlando and have family in Nashville, so we've driven part of this route many, many times. In fact, we're driving it tomorrow in the car to pick up our grandsons for a couple of weeks with us.
I checked the maps and it looks like the best way to get where you're going is I65, which you can hit in Nashville. That in mind, here's what I would suggest since you're leaving from Disney:
1. Get to the Florida turnpike and head north; make sure you have quarters for toll money as some of the booths may be unattended. Believe me, it's worth the cost to stay off of I4!
2. The turnpike will end into I75 just north of Orlando. Stay on I75 until you get to Chattanooga. Depending on the time of day, you may want to take the loop around Atlanta or you may opt to drive straight through town. I've done both in our RV with toad and without any issues. Both ways are quite a bit of traffic, but the loop around town is a bit easier to drive, just a little bit farther. Just follow the signs that point you to Chattanooga or Nashville.
3. When you get to Chattanooga, you want to get on to I24 North and follow it to Nashville.
4. In Nashville, you'll want to follow the signs to I65 North, then on to the north lands.
Good luck - drive safe. The fools on I75 can be total idiots, so set your speed and keep to the right side. If you have problems, get as far off the roadway as possible. For some reason, it seems like every time someone pulls off on the shoulder on I75, some moron comes along and plows into them full throttle.
2015 Legacy SR340 360RB
2014 Jeep Wrangler Unlimited Toad
Gigi, Poppy and Sadie Lady...On the road, but not full time!