This is one of those lists built from reports by the people who use it. An Elks lodge won't be on the list of providing RV parking for members unless a member reports it.
Although there may be accommodations for visiting members at some facilities of other fraternal organizations, most do not have programs for membership RV parking in the same way that the Elks do.
Some hospitals, police stations, city offices will allow overnight parking if one asks. Gas station owners will accommodate sometimes in the midwest and on the plains, to the extent that independent operators still exist with all of today's convenience store chains. Yet I would not expect any overnight parking list to have found them all.
You will find the same with commercial RV parks. Overall, the parks in available listings probably represent less than half the locations that have at least one or two spaces to rent out overnight. RV sites in mobile home parks and at hotels are common, but few RV park listing services will list them, because they do not meet minimum standards for a listing or expectations of RVers who buy the listings.