Here are some sites we liked for our 40' motorhome in North Campground in Bryce. Loops A, preferable and B are where you should head for a big rig. Sunset Campground has small sites.
North C.G. in order of our preference: A26, A11, A30, A24, A28
If you can't find anything there and I'd try to be there early morning and a couple days before Memorial weekend - we really enjoy Red Canyon campground, Dixie national forest on Hwy 12 about 5 mi. west of Bryce.... some nice hiking there, also. Sites we like for a big rig: 28, 33, 32, 31, 16, 30, 35, 1, 2, 6, 30, 3
We've returned to favorite campgrounds many times over the years of our full-timing adventure and we don't make reservations so I have an extensive list of our favorites - like 300 pages for the U.S., Canada, Yukon and Alaska. :)
I just thought of something for reservations - perhaps you're too early to make them for your date.......