Parking IS available for travel trailers/RVs!
Clean, convenient location right off of I-84, easy access to Multnomah Falls, trails and snacks at the visitor center. When we traveled, reservations were required to enter (can stay as long as you want once you are in), obtained from Recreation.Gov for $2.00. My only complaint was that all of the research I did from the various websites for this location (Recreation.gov, Oregon State Dept. of Transportation, Good Sam Camping Blog, Campendium/Dyrt/RV blogs, as well as Oregon and Portland tourism sites) never explained the parking situation for RVs and trailers clearly. Caused a lot of unneeded stress. RV parking is available in an adjacent lot to the car lot (no pull through spots, though).
Month of VisitJune