6 to 8 weeks near (150 miles) Seattle

TenOC wrote:
4 to 7 days per campground

In that case, once you get out of the immediate Seattle area there are lots of camp grounds, both NFS State forest, and private. The west side of the state boon docking in remote areas is difficult to find. It is mostly tree farms and the roads are all gated.
And the ones that aren't, are not suited for a 30' 5Ver.
Most all the rest areas in I-5 have dump stations (they are marked)
August time frame it may be a bit crowded.
If we can help while you are in the area, PM us.
The state has what is known as a "discover pass" it is 30 bucks, but covers the entrance costs to all state parks. These all have a $10. fee per day. I'd suggest you get one.
Most all state parks are reservation required, and have an on line web page for this.
August, is a very dry month so be prepared for forest fires, and all that entails. 400,000 ac burned last year alone.
I'd second the trip around the Cascade loop, but with a trailer in tow, I'd do it west to east starting in Arlington Wa, Rockport, Marblemount, and over the pass. doing it the other way puts you down hill into some pretty sharp corners, the west side is one big long grade. and from the top is a great view that you miss going east. Do the pull out there is plenty room to turn the trailer around and take the little walk to the over look.
Have a great trip.