There are quite a few campgrounds within an hour of Fresno off Hwy 41 and the south entrance to Yosemite. You can try Lupine and Cedar Bluff at Bass Lake (USFS), the Yosemite RV Resort in Coarsegold and High Sierra RV in Oakhurst. However, I would recommend going further North and stay near Groveland or Jamestown. Depending on what you are looking for, groveland and Jamestown have a bit more historical points of interest than Coarsegold and Oakhurst. The access to low elevation outdoor activities is better up North too.
I lived in Yosemite Valley for almost a decade and have seen weather range from heavy snow to warm and sunny in April. You should most likely be fine though. I think you might have a higher likelihood of dealing with road closures from rockslides and flooding when you visit. Usually, NOT ALWAYS, access to/from Yosemite Valley, is resolved in a day or two.