That's quite a drive, should be fun. Hot is an understatement. It was hot when I went there for Christmas - I can't imagine how it would feel in August.
If you don't like mountains, I think I 40 does it's best to avoid them. I 80 or I 90 West out of Illinois hits a lot of mountains once you reach Colorado or Wyoming. I 40 to eventually I 10. It's going to be very hot along the entire drive, so make sure your on board systems are ready to work pretty much non stop.
As far as parks in Death Valley, it's a big place. When I think Death Valley, I think Furnace Creek. There's a small RV resort there, privately run. It's next to the National Park campground which has no hookups, but is also very nice.
