To classctioga73: what CloudDriver said about rental cars. I strongly suggest it. If you are camping within the park, you can still get a rental car and park it near/in your campsite. It is possible to drive around the park in a 30' MH and it is possible to find parking spots, but perhaps not when you want to be in a particular place. Sometimes it's even difficult in a car, if it's a popular stop.
Thanks, 4runnerguy, for the additional "fun facts." I didn't know that about Death Valley! Yellowstone is the oldest (and some may argue, the best :) ) but it hasn't been the largest NP for quite some time, especially after Alaska joined the Union.
Sunset in late June/early July is 9:15 to 9:30 PM, and it remains somewhat light out until almost 10 PM on clear days during that timeframe.