Yosemite...we stay at the Thousand Trails CG about 5 miles from the entrance. The campground on the north side is open to the public. They also accept RPI. Late August is good, some schools are already back in session. Labor Day is a zoo...
Redwoods....be sure to take the Avenue of the Giants tour. That road will take you through the most awesome redwoods you'll ever see. Plan on spending at least a day or two to stop at all the turnouts. Short trails wander through the trees with informational signs posted along the way. Very quiet and serene. Along the Avenue of the Giants, stay at the Ancient Redwoods RV Park. Big rig friendly and nice owners. Unless you have a small Class C, you'll have to use your toad. RVs are not recommended. Just north of the RV park on Ave of the Giants is a great roadside stand for produce...don't like what you see on display, tell them. They will invite you to their garden and let you pick what you like...doesn't get any fresher than that....!
Another gem of a campground is the Klamath River RV Park. Stay away from Kamp Klamath. Klamath River is before Kamp Klamath, is newer and nicer. Grassy sites, no gravel, no concrete...faces the river. In September you might have some fog coming inland off the ocean. The RV park is near the mouth of the Klamath River where it empties into the ocean. Great salmon fishing..... They honor Passport America and RPI.
If I were making both parks a must, I would probably start with the Redwoods in late August, early Septembeer and hit Yosemite mid September. You'll need a light jacket or sweater in both places.......Dennis
Yosemite Lakes CampgroundAncient Redwoods RV ParkKlamath River RV Park