Boy, parking on the downside of Sylvan Pass makes for a beautiful, but long, commute each day you want to go into the park. I would echo the folks who suggest trying to find a campground or park in Yellowstone. It will probably take you over an hour to get into the park from the Valley RV Park and at least an hour to get back to your trailer. However, that's not really too bad as you would enter Yellowstone at a junction that will allow you to tour each of the loops and see some wonderful scenery. Just take it easy and enjoy your trip, don't worry about the time. And, by the way, make sure you take time to go see the Buffalo Bill Museum. It is a wonder and will take you at least one full day to go through it all and enjoy it.