I have always hiked the MW trail in either late August or very early Sept, and I have always stayed at the Portal Campground making my reservations using the Reserve America web site.
Apparently you did not read the post about the
Recreation.gov changeover 2 years ago. Reserve America took over the NPS reservation service and merged it with what you once knew as
ReserveUSA under a new contract. It is now called
Recreation.gov and the reservation window is now 6 months in advance to be consistent with the Park Service. ReserveAmerica also handles the California state park reservations.