Beautiful and cool camping is available all over the Inyo National Forest in California. Head on up around the Mammoth Lakes area.
In Colorado, you can't go wrong heading to Silverton in the heart of "The Switzerland of America" (the San Juan Mountains).
It's always nice in the Shoshone National Forest of Wyoming this time of year too.
Here are some maps and more information...
Free RV Camping Near Mammoth Lakes, CaliforniaMammoth Lakes Area RV Camping InformationRV Camping Near Leadville, CORV Camping Near Lander, Wyomingedit: I see I put Leadville instead of Silverton so here ya go...
RV Camping Near Silverton, CO