I'd suggest getting to the campground earlier in the afternoon the day you check in. Last summer I got to Bridge Bay early in the afternoon, drove to my site, and it was impossible to get level in. I had to go back and beg for a different spot.
What type of RV are you renting? A Class C motorhome (guessing from the length). Do you plan on driving that around each day or do you have another vehicle? I ask because the length of some campsites is for all vehicles. I camped in Madison Junction a few years ago and had to park the second vehicle a long way away.