I think the camp at Convict Lake has some FHU spaces. I've stayed in the lodge there a few times and it's a gorgeous setting about halfway between Bishop and Mammoth. I've never looked into camping there, but I always see people in the CG and it looks pretty decent. The fishing is good and the restaurant and bar at the lodge are incredible.
I've stayed at the RV park in Mammoth and it was OK but not great. I think our space was water & power only, so we had to roll the tote down to the dump to empty out. It's never fun to use the tote, but it's not the worst solution either.