I have used both the online version and the print version. I have found it to be useful, and worth what I paid for it. I have stayed in quite a few of the RV parks they recommend, including some of the ones where they provide a discount coupon with the book. But they only show RV parks they have personally visited, and therefore the selection of parks is somewhat limited. There are far more RV parks that can handle big rigs than they show. But you can be sure if they list it, you will fit!
I think they consider big rigs to be around 40 foot, and will show which spaces specifically they consider to be good for big rigs. And they will assume the park has 50 amp service, or they will note if it doesn't.
I also make extensive use of rvparkreviews.com. They have user reviews and include a 1-10 ratings scale. They show whether parks are suitable for big rigs, whether they have 50 amp service, etc. And they include links to a map showing the location and a link to the RV park website. You need to take some of the user comments with a grain of salt, but they are often helpful. Some reviewers will identify parks that have a lot of permanent residents, and if they are tight to get in and out of. The website is very easy to use, and will show clearly other nearby locations with RV parks so you don't need to go back to a map to see where else you might want to look. And it is free. Can't beat the price! This is my number one source now.
Back in the old days when I had a class C coach, I just used AAA campground books. I was sad to find out they are no longer producing them, or at least many of them.
'04 Alpine Coach 36' MDDS