Check out Fort Casey State park, right across the way from Keystone. Our good friend and photographer Pat G (not on this board) stays there all the time when diving Keystone. She goes most good tide weekends when she isn't too busy working!
Deception Pass is also very nice, not sure you can squeeze in a trailer of that size there though. But if you plan on taking it slow and getting fills @ Oak Harbor, it may be an option.
Hope you get to go!