When I built mine, used 8 inch fiber mix with rebar, in the area where the MH sets. I tapered it to 5 inch to the sides of the garage. And added sewer, water and power. Garage is 28x48 with 16.5 ceiling and 14x14 door. MH is very happy with it. Also put down 3 layers of 6 mill plastic over gravel before pouring concrete, to keep moisture down. Have taped down a 3 ft sq of plastic on floor, with no moisture after weeks of being taped down. Wanted my concrete to hold with jacks down. No cracks after10 years. Good luck with it.