Hey Richard, you have 3 options for camping at the Portal. Depending on when you arrive, 1) you can snag a place at the packbackers walkin site; 2) find a place at the family sites (this might be easier then you think since kids are back in school and most people (regular campers) are done for the season); 3) the one that you mentioned.
As I mentioned, I suspect the family campsites (half are not reseverable.... first come) will be a good option.
Paul