The Olympic Peninsula is where we first ran across a GTRV rigged up with solar, and started this whole obsession in the first place!
Spend some time on the wild Pacific coast (Kalaloch and north), it's a completely different "beach experience" than I've had anywhere else. Don't expect to go swimming, you'll freeze, then get pummeled by a rogue log
Long Beach and Ocean Shores are tourist traps, skip them unless you really want to do some beach driving. Cape Flattery / Neah Bay, in the Makah Reservation, is worth a side trip if you're not rushed for time. Everything in the Park is great (beaches, rainforests, and mountains, oh my!) and worth a side trip, except for Sol Duc Hot Springs, which has been over-developed to the point of just being a smelly kiddie pool.
I don't know much about dispersed camping (yet!), but it's not allowed in the Park proper, and campgrounds in the Park can be tough without reservations... less so this time of year, but still a challenge on weekends, and most are uncomfortably dense.
The Olympic National Forest does allow boondocking; I'm not sure about the various Indian Reservations, it's worth asking. Please record the lat/lngs of any good spots you find and send them to me!