Well, we don't know for sure but since the O.P. has a motorhome I'm going to assume there's also a toad, and that's what will be getting them to the kayak trip jumping off point. There's parking at the ferry landing if they want to walk on, though I'm not sure how long vehicles can be left there. And if they drive on they'll likely still have to wait in line, but it
might be for a shorter time than with the moho. Though I believe those are first-come-first-served just like everyone else.
My recommendation in that case for a land-access home base would be
Deception Pass State Park. Only fifteen minutes from Anacortes, in a gorgeous location, and likely to accommodate the rig.
Reservations recommended- it's one of the most popular Parks in the State.