Theres a few in the Grampians area, not all accessible for vans of all (or for some, any) size. Call the National Park office in Halls Gap - they'll be able to give you more info. You'll also need to buy the camping permit from them ($15 or so I think)
There are a couple of spots around Mount Zero/Mount Stapleton area at the Northern end.
Buandik Campground - in the Nat Park but was closed after the floods. Not sure if reopened. Also might've been small vans only, can't remember. Glendinnings camp in the west (south of Black Range state forest) camping permit also
South Mill Campground, near Lake Wartook. Check about vans gouing up mt victory road though
Jimmy Creek - In the Nat Park half way between Halls gap and Dunkeld. Don't really know anything about this one.
Buandik Campground - in the Nat Park but was closed after the floods. Not sure if reopened. Also might've been small vans only, can't remember.
Borough Huts area I think there's one too
Sorry - all a bit vague as we stayed in the c'van park last time and cabins before the caravan days.