Crater lake, Yosemite, Kings Canyon, Death Valley

Why not just go and not make reservations? You won't have problems at that time of year. Then you can monitor the weather as you get nearer to those parks. It makes for a more carefree vacation if you don't make commitments with reservations. :) We rarely made reservations during our 16 years of full-time travels.

I don't think you'll have problems with snow in September and no, you don't want to have to worry about chains on a RV. If that would happen to occur - which I really don't think it would - you just spend an extra night in a campground until the roads are clear. In Sept. it's not going to stick and build up.

Go and have a great trip!

Typically the Cascades and northern Sierras do not see snow until late November. But, many attractions close after Labor day, so if your plans include anything other than driving and taking pictures you could be a bit late

Weather.gov and check the area history.

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.

It all depends on the winter snowfalls. When we were at Crater Lake in July only half of the loop road was open. St Helens was still snowed in at that time. But other years the roads may be clear in late May. All you can do is watch the park's websites for information on when they think the roads will be clear.