Go to a parking lot that is wide open. Pull up next to a stripe on the ground just like it would be the center line down the highway. You want the stripe to be parallel to the motor home. While you are sitting in the driver's seat, in the normal driving position have someone outside note how far away from the stripe that you are. 12" is good. Then have that same person use a piece of black tape to put on the bottom of your windshield that corresponds with a point 12" to the right of the line, as you are looking forward.
By using this piece of tape, as you drive down the road, you will be able to tell exactly how far to the right of center line that you are or exactly where the left side of your coach will be going as you drive in an RV park or other unmarked area.
With a large vehicle, you will go crazy trying to watch both sides at the same time for clearance. If you are sure it will fit between two spaces, all you really need to do is watch one side and the other side will take care of itself.
Basically it all boils down to experience. The more you drive it the more comfortable you will become. Relax and enjoy the ride.