Even if all the sites are reserved now, cancellations can happen. If you really want to camp in the valley, check back a few times per week on the reservation site and you might hit an opening that will work for you. I had a specific 5-day window I was trying for during one July, and just a couple weeks before departure I snagged two different sites (one in North Pines, one in Lower Pines) for a Fri and Sat. On Sunday am I got up early and drove to one of the first-come CGs (Bridalveil, which won't be open in April IIRC) and easily found a site for a few more days.