Washington has tremendous boondocking opportunities, and you don't have to cross the Cascades to get to them. The problem you will find is heavy competition for good boondocking sites.
For one example, if you head east on highway 20 north of Seattle, take Baker Lake Road north near the town of Concrete. US National Forest lands are open to boondocking and there are several USFS campgrounds that work great for base camps.
US Forest Service - Washington Official Travel Maps
- is a page you should visit. Each national forest is developing new travel plans and you MUST have one of these maps if you want to stay out of trouble camping. The maps show open roads, and these roads are open generally to dispersed camping (boondocking) as well.
The ranger station can be your best friend when looking for boondocking locations! Ask about dispersed camping sites they are aware of. We have been directed to some of the best camping sites just by asking at a ranger station. We have also been given some questionable advice since our rig was probably too big in the past to go to some of the places we were directed.