Festus gave you excellent info & advice, but there's one thing I will add ---
Over-night stays in parking lots (WalMart, malls, etc) has been banned in 'some' cities & towns across the U.S.
Some still allow it, and others don't, depending on their current local ordinances. I'd hate for you to drive 12 hrs,
only to be told to "leave" the parking lot... so, it might be best (if you still have time) to email the Chamber of
Commerce in Everett. Yes, it's always best to have a *Plan B* & especially when traveling without reservations.
Here's one (prohibited WalMart parking) list, and there may be others:
http://www.walmartatlas.com/no-park-walmarts
We've found that sometimes the 'much smaller', more rural, communities will allow over-night parking at their city parks...
some are free, and some charge as little as $5.00 for electricity (only). Check these websites for some ideas --
http://www.freecampsites.net WA
http://www.freecampsites.net/attribute/states/wa
http://www.freecampgrounds.com
I agree with Festus -- given that it's the height of summer travel season, you'll likely find that Campgrounds & RV Parks
near popular tourist attractions, are 'full'. Reservations are typically booked weeks/months in advance, but (obviously)
cancellations 'do' occur, so check for availability. Also be aware that some State & National Parks only do 'online'
reservations now, and their websites should give specifics.
Hope this helps, and enjoy your trip.
Terri, the Chevy co-pilot