Cold air falls. By skipping the floor, you create an endlessly hungry "cold sink". Concrete floors are a conductor (not an insulator) so no R value is gained there.
If you are pouring a concrete pad for your cooler, please use between R25 and R30 styrofoam underneath the concrete. You will have the benefit of the insulation and the easy maintenance of a concrete floor.
The only time you can skip a floor is if the ground temperature is less than 13 degrees warmer than the desired temperature in your cooler. (Do you live in Alaska? Are you only using the cooler for hunting when the ground temp is 53 or less? Do you only want the cooler to chill to 48F?) If you up size the air conditioner, you may be able to reach your desired temp, but it will cost you more in electricity and wear and tear on your air conditioner.
A truck or trailer will never cool properly without a well insulated floor. The cold air inside falls and heat from the road rises and warms the floor from the outside.