Siyonara, Japan.
I dunno about tents for an April Yosemite. Lodging seems the better way to go. About the only place I can see the Lashes being able to park their RV is Curry Village. There are cabins there that are charming, and canvas tent cabins that are cheap. Alas, the RV won't be able to park overnight at Curry Village ... and probably not at all anyplace else.
Perhaps the Lashes should reserve a campsite for their RV.