Good idea to shorten your stay by one day to get a site. I think that's what frustrates those who try to make reservations...they're set on specific days and when they plug those days in and get 'none available', they don't try to play around with those days. Many times you can also click on a specific site and then click on the previous two-week period or the future two-week period to see exactly what days are available.
If it's a park you have your heart set on perhaps two or three days would be worth it to you rather than not spending any time at all there. You can always go to another nearby campground for the remainder of your stay. The key is to be flexible.
We've volunteered in public parks a lot and we know that many have a certain percentage of sites that aren't even reservable. So sometimes if you're already in the park under a reservation and you want to stay longer, check to see if you can move to an unreservable site. This is especially true of national parks and national forests. All sites are not reservable. Safe travels!
Full-Timed for 16 Years
.... Back in S&B Again
Traveled 8 yr in a 40' 2004 Newmar Dutch Star Motorhome
& 8 yr in a 33' Travel Supreme 5th Wheel