The camping is kind of thin in the Snohomish area. The only place close to Snohomish I know of is Flowing LakeCounty Park. There is a Thousand Trails affiliated cg just south of Monroe called Thunderbird. Another option is the fairgrounds in Monroe if it isn't during the fair that is.
Most of the camping is much further east, Skykomish and beyond.
Did you check with the tournament organizers? They might have a parking lot at a near by school or parkavailable. Whenever there is a big tournament at the Fairfield soccer facility here in Monroe there are a lot of RV's set up for the weekend at the adjacent school.