I don't know what MH you have, but you could easily have a 6000 lbs front wheel load spread over 50 sq inches of slab trying to punch thru the new concrete slab. Given good curing practices and no excess water added to the mix, the concrete should reach it's design strength in 28 days. If you have 6 inches of heavily compacted gravel under the slab on top of good original ground (no fill, only cut) you could probably be safe after 14 days of wet curing as described above. If you don't have the compacted gravel and soil I would recommend you wait as close to 28 days as you can and keep the slab wet (burlap, plastic, or a tarp will help) during the entire time.