I was just up there today. The road isn't all that rough, as in potholes, but is very washboarded for miles. If it's like today you'll surely see many cars (even a Prius or two) on the road headed past Evans up to Mowich lake. I just took a quick peek into the campground, I didn't see any motorhomes, but did see some pretty long trailers. I wasn't looking with a RV in mind, but I'm thinking that you might rub some branches getting to the campground in something as tall and wide as a 38' RV.
Sorry, I can't say absolutely yes or no on getting in to the campground with your rig.