The Twin Lakes campground looked really nice but extremely crowded.
The June Lake loop campgrounds are also the same. (Very pretty scenery but crowded).
I usually stay farther north or south of Mammoth to try and avoid the crowds.
One little campground we liked was near Lake Crowley. I believe it was a USFS campground called McGee Creek.
The area is next to a small creek with difficult access for fishing. It overlooks Lake Crowley. I watched a 40' MH come in and sandwich into one of the sites so it can be done.
My smaller size MH made it in with no problem.
Convict Lake isn't bad either. Very nice setting and scenery. Boat rentals, hiking, fishing. Campsites are a little tight but if you have a 26' toy hauler it should be ok.
Ed