Crater lake, Yosemite, Kings Canyon, Death Valley

The Sierras can be unpredictable but you should have no problem with Yosemite/Kings Canyon/ Sequoia in Sept. In fact, that's a great time to visit after summer crowds. Yosemite Valley is at around 4,000 feet. Normally it's late Nov before snow accumulations and more likely late Dec to March.

Grants Grove in Kings Canyon NP is at 8,000 feet but still should be good in SEP. we visited all winter as locals, just didn't drive during active storms. Did not RV in winter though.