George, I don't know the specifics on your wiper setup - so I'm guessing at some of this. Maybe someone else will be able to provide more information.
In my experience (working on older cars/trucks) the park function is built into the wiper motor and can not be adjusted. If the park function contacts are dirty you might be able to clean them, but other than that, you would need to replace the motor assembly.
This is a diagram for my Ford truck. I believe it is similar to the wiper motor setup in our coaches.
When the wiper switch is turned off, there is still power applied to the "park contact". If the wiper arm is not in park position (vertical), power still flows through the park contact and keeps the motor running. Once the wiper arm reaches park position on the windshield the electrical park contact is no longer in contact with power and the motor shuts off.
So if the park contact was dirty or worn out, once you shut off the wipers power to the motor could be interrupted before the wiper arm reaches park position. This would stop the wiper arm before it parks in the correct position.
If you can tell me the make/model of your wiper motor, I would be happy to continue researching this.