Every park is different. Some will have lots of sites that can accommodate big rigs, some will have just a few. There isn't any rule of thumb that you can use. I can tell you for example that most state parks in WI have sites that can accommodate a big rig, but some of the parks only have a few large sites like this.
Many state and federal facilities are reservable through
Reserve America. They usually list the lengths of the pads. Another resource is
RV Park Reviews. A lot of folks with big rigs will often make comments about a given park's ability to accommodate big rigs, state and federal CGs included.
Chuck D.
โAdventure is just bad planning.โ - Roald Amundsen
2013 Jayco X20E Hybrid
2016 Chevy Silverado Crew Cab Z71 LTZ2
2008 GMC Sierra SLE1 Crew Cab Z71 (traded)