Thanks for all the replies and I will use the info to better quiz potential contractors.
I'm not going to spec anything but size and usage. I don't want to be put in the position of a contractor saying..."Well that's what you asked for".
I do wonder why so many use a gravel base. I sort of figured that since my ground is so hard it the slab would just be poured right on to of the soil. The only excavation I see necessary would be for the footers.
If the top of the slab is set so that the desired thickness is at the high spots of the site wouldn't that even eliminate any leveling???
FYI, my ground is so hard that driving a MH on it barely leave tire marks and will even support the jacks.
Thanks again for the info.