To your original question, I think a basic laser level would give you a pretty good idea (it won't be exact, but would let you know how tight it is going to be). I'm having trouble figuring out how to write this, but, I'll try anyway. Hear's what I'm thinking...
Measure the height of the clearance at the rear bumper and the length of the overhang. Place a board at the lowest entry point of your driveway that is the height of the clearance you measured. Point the laser level towards driveway and measure the distance between that point and where the laser hits the driveway (that would simulate the rear of the camper backing up the drive). If the distance between the board and the laser hitting the driveway exceeds the length of the overhang, you should be clear.