Put the heavy cubes in first, and get 'em packed tight so that they can't shift about at all. Sometimes laying 'em flat, face down, is worthwhile. You'll want the weight in the middle of the van, not to one side, so balance 'em out. Anything 'crushable' (drums..? cymbals..?) goes at the very top. Mic and PA stands can often slide under the seats, or in the foot well. Once all is in, cover the lot with a thick blanket, or your drum rug. Nothing should be able to swill about; all must be wedged against something else. As said above, do a couple of trial runs; it often happens that, perfectly packed going out, it becomes an impossible Chinese puzzle after the gig. Things seem to expand..!
For a 'normal' sized band, there should be plenty enough room. For a Genesis tour, it might be a tight squeeze.