Joane and I are staying at Battle Ground State Park in site #1. This park looks nice on line and we have friends who live out there that have visited the park and recommend it also. It only has some water and electric sites and a few big rig sites but generators are allowed. The pdf description says 35 ft limit but that is not correct as there are some larger sites. It also has cabins and tent sites. It is about 25 minutes from rally central. COG will not be making reservations so you are on your own. Right now the park appears to be fully available.
https://parks.state.wa.us/472/Battle-Ground-Lake
https://parks.state.wa.us/223/Reservations
https://parks.state.wa.us/472/Battle-Ground-Lake
https://parks.state.wa.us/223/Reservations