Idyllwild is big truck friendly. Mostly wide maintained forest service roads. Easy traveling with good scenery. I have been down Bee Canyon, Thomas mountain road, Rouse Hill Road, Table mountain truck trail, all over basically. Lots of trails plenty fine for a full size. Bee canyon has some tight spots, especially if you meet oncoming traffic, but the rest of the trails are pretty good. There are some free yellow post campsites there too.
Another area to check out up north is Upper Coyote Canyon. Its a little bit tougher of a trail, but is full size friendly. Its on BLM land so you can camp anywhere, just ensure you have a fire pit. They don't allow ground fires.
South of that anywhere is Anza Borrego is cool thousand of trails and you camp pretty much anywhere. I really enjoy the Fish Creek mountain area as well as the bad lands. No one is ever in the bad lands, and it gives you a real since of being "off the beaten path".