I have a RV buddy that lives on Whidbey island. I asked him for you....here's his comments.
As far as I know South Whidbey does not have any RV Parks. There are some
state parks, but they are older, smaller and only have water and
electricity. I don't think any of them would be big enough for a Redwood or
Montana, but here is a link: http://www.parks.wa.gov/841/Northwest-Region
All of the other places are located on the Northern end of the island. If
he is retired military or has a DoD ID card he might want to look at
Cliffside on NAS Whidbey Island:
this is a new park (3
year approx) and is located right on the sound. Then there are 2 other
parks, one is the Staysail City Park located in downtown Oak Harbor:
and the other is at the very
Northern tip of the island by Deception Pass state park:
Either Cliffside, Staysail or North Whidbey would fit his needs, but will be
about a 45-60 min commute to the south end of the island.