Other than what's been said, and I recommend Ivylog's suggestion, we love Portal RV Resort in Moab. If you decide to stay there, be sure to ask for the owner's section. It's a little more expensive but worth the extra bucks. Plan on two days minimum for the Arches National Park.
Hwy 128 out of Moab is an easy drive along the river. There are several places where you can camp right on it's banks...Click
HERE for some pics of Utah 128. There's even a dude ranch along the banks of the Colorado...Dennis