You will definitely need a reservation for the Coupeville/ PT ferry. By then, they should have a two boat schedule.
As far as routes, the PT ferry is midway on Whidbey. I recommend avoiding the Mukilteo Clinton ferry and driving around through Deception Pass. It's a scenic drive and one less ferry hassle. Depending on the time of day, there can be long backups, although on July 5, most of the heavy traffic will be leaving the island,
One option is to camp at Ft Casey and take an early ferry on Tuesday morning.
Also, check the fare schedule for the cost of your combined length with passengers versus disconnecting the Ranger and driving it on separately. Sometimes it can be a little cheaper that way.
From the eastern shores of Whidbey Island, have fun and a safe trip.