Depends on how early you are on getting a site number. Check in is at 1:00 and when we were there this year we didn't get to check in early. If it's not busy, they may let you check in early.
Re-reading this thread, I think that there could be some confusion about how this could actually go for the OP.
If you arrive before 1 pm on the day of your reservation, you can check-in. That means that you go ahead and make your final payment, and you will be issued your parking pass and KTTW cards. If your site is ready early, they will give you your site number then, too, and you can go right to it and start setting up camp.
But, if your site is not ready early, you are still checked in, but you will have to park in the overflow lot, and find something to do until your site is ready. You cannot drive into the Fort, as there is no place for you to park other than your site(which you won't have the number of yet) or the overflow lot. The procedure used to be that they would give you a phone number to call after 1 pm to get your site number. I think I heard somewhere that they may now be offering to send you a text message with your site number, but I have not experienced this first hand.
And, also, I should have mentioned to the OP that when she first arrives at the Fort, she will also have to give her name and present a photo ID to the guard. I have observed that they cross names of those with reservations off of a list when each guest arrives. If you are arriving separately, you should tell the guard that, just so your DH's name is still on the list in case of a shift change, or list update, or whatever.
TCD