I've been in the Seattle area since 1979. But, I don't vandwell here, and I'm one of those folks that likes rain.
I live about 30 miles east of Seattle at the foothills of the Cascade Mountain Range. If you are talking about Seattle proper, then I would say that it's like most large cities where overnighting in a vehicle is discouraged, if not illegal. So, you've got to be real good at being stealth, or overnight in one of the neighboring towns. For example, the Walmart in Renton allows overnight parking, last I checked.
As for the weather, you'll find lots of overcast skies and drizzly rain. One winter we had absolutely no sun breaks for more than 4 months. It doesn't snow a lot; but, when it does everything shuts down. The snow quickly turns to ice and driving is treacherous. With budget cuts less roads are getting sanded or plowed too. But, we've not had any snow for more than half the winters since I've been here. It does get below freezing, mostly at night, but rarely into the teens. The daylight hours are very short in the winter too.
But, in the summers the days are very long. For example, the sun sets here at 9:10pm right now. Usually the summers are beautiful with the average daytime high in the 70's, and rarely reaches into the 90's. We still get some overcast skies and rain in the summers. In my eyes, it's hard to beat the natural beauty of the area. It's right on the ocean (Puget Sound) with the Cascade and Olympic Mountains on the east and west. Three spectacular National Parks surround Seattle -- Olympic, North Cascades, and Mt. Rainier.
If you don't need to live in Seattle proper, we've got several National Forests for dispersed camping, with Mt. Baker-Snoqualmie NF being the closest.
Also, do a search for other threads in the forums that discuss Seattle. You'll probably find some additional information.
Suanne ... likes the rain, but still getting ready to start snowbirding in the winter