If you don't already have reservations inside Yosemite it may be too late. But if you can't stay inside the park I like Yosemite Pines RV Park on Highway 120. It's about a 22 mile drive to the gate and another 25 to the valley, but you can also access the Tioga Pass area easiest from 120. First week in September most schools in California are back in session to it will be less crowded then with the exception of Labor Day weekend.
The only bad thing about visiting Yosemite at the end of the summer is the waterfalls may just be a trickle of water by then.