We just finished pouring 40 yards today. It is an RV garage floor, so that includes the front and rear aprons. I used 4000psi, fiberglass strand mix, with welded wire over 6 mil poly, and the slab is 6" minumum, with 7" in some areas.
I poured a similar slab for an RV pad alongside the house, about 10 years ago. Not a single crack in it, and I have used my jacks both with and without blocks, many times.
If the base over which the concrete is poured, is rock solid, there should be no problem with cracking, even with less than 6".
USAF 1965-1971; USCG Master
2002 Horizon 36LD - 2000 GMC Sierra 1500 4x4
Indian River, De