I tend to agree about the merits of boondock camping around Mammoth. We're up there probably once a month and there is a nice quasi-legal campground on the south side of 395 about 4-5 miles south of town that we enjoy. I can send you GPS coordinates if you like as there is no signage. There are also nice places to camp off of Benton Crossing road which heads northeast about 5-7 miles south of 203 turnoff from hwy 395.
My favorite USFS camping is in the rock creek area. The turnoff for rock creek is at Tom's place, about 15 miles south of Mammoth on 395. Big meadow and Iris meadow campgrounds have creekside campgrounds and are relatively small with a nice secluded feel. The scenery in the area is excellent. Avoid the bigger campsites down the the rock creek valley for the same reasons noted above regarding Convict lake. Lots of big RVs and noise.
Laurel lakes is very scenic and the road up it a challenge. I've done it many times on the dirtbike, but not yet in the SMB. Jeff (canyonclan) has driven his SMB up and assures me it is no problem!
And finally, on a visit in June we camped for a night at the coldwater or coldcreek campground near Lake Mary. It was a very pleasant surprise. Many of the campsites are creekside and it is a short walk down to Lake Mary (wife caught a 3.5lb Alpers so she was very happy). It is a large campground and I was concerned about crowds, big RVs and noise. Not so. Per the nice and helpful campground hosts, occupancy rates are down a lot recently so the campground was pretty empty, I'd guess about 30%-40% full. No doubt a function of fuel prices. That may change in the later summer months so check it out first.
Enjoy the trip. We're heading up in a couple weeks and I'm counting the days.