Sue, How rocky is your soil? Could you use a track hoe to dig the hole, yes i know that you are trying to do this as inexpensively as possible but you can rent a TLB or a excavator for $300-500 for a weekend. And you can not only dig the hole you can use it to put the tank in place. Of course if the rocks are the size of cars this still is not a good idea. If you decide to do this be sure to call your local underground locating service before you dig.

If you cover the tank with concrete can you put styrofoam or some other insulating material between the tank and the concrete. Don't know if the concrete has enough insulating value to keep your tank from freezing where you are at. I have seen structures made with made with styrofoam blocks and concrete on the outside. The concrete adheres well enough to the styrofoam to be shipped, stacked, and the blocks joined together on site.

For your thin concrete work are you using fiber admixtures to increase the strength? Or are you using just portland and sand with just enough water to achieve hydration?