How to calculate the requirements of a concrete slab?
Assuming your containers are 10’ wide x 40’ long and weigh 15 tons, that creates a bearing load of: 

30,000 lbs. / 10’ x 40’ = 75 lbs. per square foot (DEAD LOAD).

Add 50 lbs. per square foot LIVE LOAD (40 is minimum Code requirement for residential construction). 

So, TOTAL LOAD is 75 + 50 = 125 lbs. per square foot. 

A 4” concrete slab can easily support 125 lbs. per square foot without rebar, but you’ll need “control joints” or the slab will crack. OR, you can add reinforcing steel. This reinforcing steel is called “temperature steel” and is not used for spanning soft spots, etc. I’d use #4 rebar at 12” on center each way in a 4” slab. (I don’t like wire mesh.) I’d also try to keep the slab as “square” as possible. No jogs, etc., because it will crack at the jog. 

However, if the containers are going to be used as “habitable space”, you’ll need to install perimeter footings down below the frost line. Depending where you live, the frost line can be anywhere from 12” down to 48”...or deeper if you’re at the North Pole. 

Yes, concrete slabs are usually installed on moisture barriers (to keep ground water from coming up through the slab) and on a gravel base (to even soft spots out which cause differential settlement). The size, type and depth of gravel will depend on your location. 

If you’re located on a hillside, (you say you have to level the site,) make sure ground water is diverted around your site. 

If your soil is mush, like where I live, you can’t use this type of construction (slab on-grade construction). You’ll need to install piers or piling and design a “structural” slab to span from pier to pier.