Another vote for the mrohr approach. Doesn't sound like an option where you are, but I can strongly recommend tossing all dirty laundry into the bottom of the dinghy
, squirting some biodegradable soap on top and nosing up to a waterfall. Cover with water and dance atop the pile. Bail and rinse. Hang to dry.
Otherwise, you might consider the cost and hassle of an onboard W/D and calculate how many expensive laundromat loads that will actually buy you. May also be worth asking around for a little mom n pop pick-up/drop-off wash n fold service
, even if it's just a sweet old lady down the road. A win-win arrangement if you find someone with the time who needs some extra cash.