We managed to get a cancellation for our first night at Zion, and then went early the next morning to the other (non reservable) CG. It was easy. Actually, while walking around the night before, we found someone who told us we could have his, so we paid and put our info on the post under his. We also pulled in right after he moved out. So, keep checking the site for cancellations, and if that does not work, then you might need to stay in an RV park outside the gates. BTW, after 5pm, they give away the handicapped sites, if no one has a need for one. Sometimes they allow overflow to use the parking lot. They try very hard to accomodate people.
We did not camp in Bryce because they had one of the CGs closed, so we went to Kodachrome (A lovely Utah SP instead), again, we were lucky to find a 3 night cancellation.
Capitol Reef - if you cannot get there early, there are several RV parks in town, or places to boondock, and then show up the next morning for a site. BTW, Fruita was my favorite CG of our trip - possible one of my favorite ever. We had planned on 3 nights, but ended up staying 6. I would love to go back sometime for the full two weeks.