Some of it goes to run the RVIA trade association and fund their activities (lobbying on behalf of the manufacturers, keeping track of standards, in theory ensuring that the manufacturers adhere to the minimum legal standards...) Another (and as I recall bigger) portion of it goes to the "Go RVing" ad campaign.
The fees are paid by the manufacturers who are members of the RVIA. If they itemize it on their invoices, then that's just how they happen to account for it. I guess it sort of falls into the category of "mandatory options packages" that some manufacturers have, presumably a way of making it possible to list a lower base price for the RV than it is possible to actually buy one for.