The 'angle of repose' is not your friend with coal hoppers. I does not matter what you do unless you have an auger that dumps directly in the center of the hopper. Axeman, EFM, GJ, and Van wert all have augers, but it would take a series of auger to make you situation better. Most of us assume the responsibility of checking on the units once a day, and ash and more fuel are two of the things necessary to deal with. You can add an auger, but expect another grand at the low end and many more for a complete automation. A shaker/vibratory unit would cause issues itself due to 'fines migration'. Do not think it has not been tried, and used successfully, just understand that you may end up with more work than it may be worth. You could build a direct funnel type of hopper for total gravity operation, however height requirements may nix that plan. It cannot take more than 2 minutes to cut open a 40 or 50 lb bag and throw it into the hopper, so evaluate your priorities and then make your move.
Many times the 'KISS' philosophy is the logical answer.