When you say island, do you mean the San Juan Islands or Whidbey Island, perhaps? Because Port Townsend is obviously on the Olympic Peninsula. If so, there is a ferry service from Port Townsend to Whidbey's Coupeville. Unfortunately, no ferries for the San Juans; for there, you'll have to depart from Anacortes. One-way fares from Port Townsend to Coupeville look to be about $35 for rigs up to 30 feet in length (you'll have to pay for the tow vehicle plus each passenger, too). On Whidbey, there are a number of RV campgrounds including at Oak Harbor, North Whidbey RV Park (south of the Deception Pass Bridge), and a limited number of slots at Fort Casey State Park near the Coupeville ferry terminal. Let me know if you need additional information!