We have two favorites in the western Sierras. Both are great and it would be hard for me to choose one over the other.
On Shaver Lake is Camp Edison. Sites are great with full hookups. The lake offers water adventures and there is plenty of hiking opportunities. May will be cold and there may be snow on the ground or a possible snow storm so watch the weather.
The other is Sequoia RV Ranch in Kaweah near Three Rivers Ca. It is just outside of Sequoia Nat'l Park and can be used as a base camp to visit the park. There is a river flowing through the property with great trout fishing. It is at a lower elevation than Shaver Lake.