When looking at the sites, note whether or not there are trees or big rocks at the back of the camping pad. Our 25' Tradewind can hang over the back edge of the pad almost 5' and still have the wheels legally on the pavement.
Some of the length restrictions are due to tight curves to get to the campsite. Long bus-like RV's can't make the tighter corners without scraping a tree or two. So the foot limit may refer to the maximum length of each part of your rig.
As an example, we stayed in Upper Pines site 46 with our 25' Tradewind and 20' F250 short bed crew cab pickup. We just barely got our front bumper out of the roadway. The tree at the back of the campsite prevented us from hanging over the back as far as we might have. The tree on the door side prevented us from opening our awning, but the site is in nearly full shade anyway. The tree on the non-door side of the trailer meant that a spotter was essential in backing into and pulling out of the site. But we did fit and we had a lovely visit.
Reservations are essential, as someone mentioned. The reservation window opens once a month for all reservations up to 6 months (I think) away. All reservations are usually filled within an hour of the opening time. Check the website for details of when reservations open and know in advance which sites you will and will not fit in. I usually try to access the reservation website while trying to reach the phone reservations at the same time. The website sometimes crashes when you are part way through the process.
One can also check the reservation site often and catch cancellations. That's how I got site 46 on one month's notice for an October visit.
Sites are unreserved in the off season and there are often available spaces. But, once again, who knows if they're the spaces you'll fit into.
Best of luck. It's a lovely place. If you get there, be sure to hike up out of the valley, even a little way. Most people stay in the flat part and you can get more of a wilderness feel by going up.