First, just do it. Understand that the RV is like a house and a car. Both will need maintenance and repairs while you are living in it. So, don't let that frustrate you, it can be a pain.
As some others have said, figure out how many miles a day you want to drive, and how many nights minimum you want to stay in each stop. We use a 4-3-2 travel guide, that is get there by 4pm, drive around 300 miles each day, and stay at least 2 nights at each stop. Others use 2-2-2 or whatever fits their preferences. Then take a look at your route and figure out where you want to stop longer and estimate how long a trip you want to take. 3-4 months might not be enough time, we just took 3 months just to do the west coast from The San Juan Islands to San Diego. But that's us. We discovered we like to stay a bit and get to know where we are, and see as much as we can.