1. As stated, dealer specific. If you have a local dealer that sells the same brand you have better chances.
2. As stated, campground specific. No two campgrounds are ever the same. Private run campgrounds are handled different than State and Federal. For Private run campgrounds, you usually call and give them the length of your camper. They fit you where you fit. Some private sites have on-line registration and may or may not give the site length.
The State Park / State Recreation parks (more and more states) are using Reserve America. Reserve America will display a map of the campground and when clicking the desired site, you'll get more details and even a photo of the site. Site length, distance to water, and sewer provided or not on site are always identified too for each site.
If you take the Reserve America map and open a second browser and go to Google Earth or Google Satellite map, you can locate the same campground and see an ariel of the actual site. Between the 2 maps and the detailed information provided, you can determine if this is a site you can navigate or even want. The Reserve America campground maps don't show things like, trees and brick walls that may be on the other side of the road preventing a longer camper to back into comfortably.
So, basically, every campsite in every campground is different. Do your homework, make your reservations, and you'll have no problems finding a spot. Mine is 35.5 feet long, with my cargo carrier, I'm 38 feet long. With tow vehicle I'm 59.5 feet long. My camping is East of the Mississippi River and we've not found a campground yet we've not been able to camp at. The site might not be the one under the trees, along the river bank, with at pristine scenic sunset view, but we get a place to park somewhere.