Well you'll want to make sure the stairs don't drift from each other. So make sure they are attached. I'd say you could do a vertical rebar through each step, maybe two, or even with a wood stake if you're in an arid climate. Then, connnect the bags with wire or string. Id do a hollow deck so you can save on labor/material. Then wood frame atop. Or fill with trash, and do earth bags atop with set stone, maybe.
