It's likely to all be busy (though not to say you don't have a chance for a site.) If you've got reservations for that whole time at FB, you can probably get it changed, just take the nights out that you want to camp in/near Cody, and return. Not sure that's how they'll want to handle it - they may insist you cancel and try to rebook (which could leave you w/o a campsite in the park at all) - but it might be worth inquiring. Depending on the size of your TT, you may find precious few sites in the nonreservable (and smaller) CGs that you fit in. Something to keep in mind.