Its not the floor you have to worry about, its the walls and roof. The ground temperature inside the greenhouse will be determined by the air temperture inside the greenhouse. Sinking an insulated stem wall below the foundation will help keep cold from coming in from the sides.
With twin wall its going to be difficult to maintain an air temperature of 65. You may want to look into greenhouse bubble wrap. Heating is going to be your biggest expense and your biggest problem.
The gravel/sand/concrete block makes a good floor. Tropical greenhouses are wet places and floors that drain well are important. Make sure you have a dry spot for your heater.
Why 65? I grow orchids and try to maintain above 55.
