Unless things have changed most of the campgrounds on the BRP in the past are first come first serve basis and you cannot make reservations. Mount Pisgah is the only campground that has taken reservations in the past that I know of. This may change as the current schedule for 2015 is not been published that I can find at this time.
The area you express concern over being the Rocky Knob campground I would not be concerned about. If by chance you find Rocky Knob to be full, it is a mostly downhill 9 mile run to Meadows of Dan which has a private commercial campground which I have stayed at. It has a separate area for tenters. In general, I have found that commercial campgrounds will not turn you away when they figure out you only have a tent, do not need hookups and they can charge full price to you and stick you on a patch of grass somewhere.
I made no reservations in advance either time I did the trip, both of which were in the month of July. I did get into a bit of a jam at Mount Pisgah arriving there on a Saturday night. The tent sites there were all full and I had to pay the higher fee for a RV site is all.The only bad side of that was I told I had to pitch my tent on the asphalt pad provided for the RV and could not set it up anywhere else on the site.
If you want to stay in one of the Lodge's, Peaks of Otter or Mount Pisgah I would call ahead. Unless it is a holiday, peak season period they generally have rooms during the week but fill up on weekends.
Cell phone service can be hit or miss on the parkway, if you need to make reservations or call you might have to think about where you can get service.
On my trips I planned my itinerary for about the first three nights, after that it kind of all went to pot depending on the weather and how I was feeling. The thing about riding the parkway is you kind of need to know where to get services and hit those spots. The real mistake is having to leave the parkway for a service which might just be four or five miles away but be 1200 feet lower in elevation. Starting a day with a huge climb in addition to what you have to do on the parkway can just be killer. The other thing is that there are long stretches on the parkway with no services, when you hit those areas you need to be prepared before you try to ride them.
The Skinner book "Bicycling the Blue Ridge Parkway" is excellent for describing the terrain, less so for services. Mainly I think the 2008 recession killed a lot of the seasonal businesses that are said to operate. I see that there is now a 5th edition published in 2014 available, perhaps the information in the current one is more accurate.
I hope this helps. Feel free to ask some more questions.