In Washington state parks, the Discover Pass is for day visits. State park daytime hours end at dusk. If you are a registered camper with a paid overnight stay in a state park, you do not need to have the Discover Pass. Simply display your camping registration on your dashboard.
The extra overnight vehicle fee in state parks is associated only with camping stays. If you are camping, your registration includes a single motor vehicle for tent campers, or in the case of RV camping, the RV and a towed vehicle. If you bring an extra vehicle for an overnight stay, you pay an additional $10 extra overnight vehicle fee. If you bring your RV in and drive a second vehicle in for your stay, then you pay the extra overnight vehicle fee. Regardless of these circumstances, you will not need a Discover Pass.