Fort Stevens should be nice in October, last time we were there the mosquitos were really bad. By the time we left I looked like one giant mosquito bite.
We make a few trips a year to the Vancouver BC area and the one problem that we have to stay on top of is humidity. If your vents fans can't stay ahead of it, you might want to consider getting a small dehumidifier.
When we get home from the coast, we leave all the cabinets and interior storage compartments open and set out half a dozen containers of calcium sulfate. You can pay the RV dealer an inflated price for "Damp Rid", our find your local chemical supply store and buy it in bulk. We made our own containers out of ice cream buckets and plastic dollar store strainers. The last 50# pail of calcium sulfate that I bought cost me $50, the cost to buy the same volume of damp rid at a dealer would cost in the hundreds.