Only problem with going through Cody is that you have to pull over Sylvan Pass, which is pretty steep and high or go over Chief Joseph Highway which is pretty steep and high. I do believe that you would be going a long ways out of your way if you were to go from Billings to Cody to get into Yellowstone. Colter Bay campground does not take reservations, but the RV park does. Don't know about your needs, but the campground is great for almost any size trailer up to 35-40 feet in some pull-throughs. Your 19 footer will fit just fine. As far as getting, checkout time is 11:00 am so getting there around 9:30 or so works great for finding a spot. I recommend Jackson Lake because we have been going there, almost un-interupted, for 50 years or so. If you do stay around Jackson Lake, take the lake cruise that takes you out and around Elk Island and into the shadow of Mt. Moran, looking almost straight up about 5,000 feet. Jenny Lake is also very beautiful and offers some easy hikes around the lake to Hidden Falls and Inspiration Point. Whatever you do, don't cut your time short, if possible, and see some of the best features, in Yellowstone, in the country and some of the prettiest and most majestic scenery in the country in Grand Teton.
