What is the farthest place you ever wanted to go that you can drive your rig? Start there. Don't waste time with places close to home that you can do with weekend or even week long trips.
You say you like amusement parks Six Flags has one in St Louis (there is another Six Flags just north og Chicago too.) then on your way to Ohio Not worth a special trip from California but if you are a Elvis fan you could stop and see Graceland. When you are in Ohio I think they have the Rock and Roll hall of fame and the Footbal Hall of Fame. There is a lot of little things like that all over the country. When you decide on a main destination I am sure you will get a lot of feedback on the area and the route to and from. Four weeks is a long trip we we all know how fast it will go by.