Grand Coulee Dam

WE stayed at a campground there years ago, Sunbanks I think, and the full hookups sites were the tightest we have ever seen. The unit next to us had his slide out and it was partly over our concrete patio. We had to move our picnic table, so he could get into his compartments. If he moved further away from us, he couldn't get out of his door.
We were supposed to be there for a week, but left after two nights and got all our money back.
There is a small municipal park at the South end of Soap Lake, but that might be a bit far for you.