Many folks have given you good advice about how to actually get to the Fort.
I looked up Lascassas, TN on Mapquest.
It also depends on what type of driving you like to do with your rig.
If you like Interstate, then by all means take I-40E or I-24E over to I-75 and just head south, then get on Florida turnpike, then get on US 27S at Clermont, then follow US 27 to US 192W (Right Hand Turn On Ramp), then follow US 192W to World Drive (Right Hand Turn On Ramp), then follow World Drive straight to Magic Kingdom Turnstyles, far right lane, then take first right to Ft. Wilderness, then turn left into the campground.
If you prefer regular back roads, then that is a different story.
Carol