In most COE parks, the handicapped site may be used if all the other sites are occupied.
Here is some wordage on the reservation site:
"You are about to make a reservation for an ACCESSIBLE SITE.
This site is accessible for persons with a disability or otherwise limited physical mobility. If no one in your party has needs for an accessible site, you should not reserve this site unless this is the only site available at the time of your request. This accessibility alert does not apply to group campgrounds, group day-use shelters, cabin and lookout facilities."