[Nandy]

I had a chainsaw that will do the same thing. I would start looking at the carb. The gas can be drained there and from there move to the crankcase. Divide and conquer, how good are you wrenching? That leak is large enough it should be easy to find. A simple test will be to put gas in the tank again and leave a cardboard under it that will cover the entire footprint of the atv or at least cover the motor area. This will tell you if something is dripping. Mine will drip a bit on the petcock, an after market shut off valve and thru the carb. I replaced the aftermarket shut off valve, tighten the petcock and that took care of the carb leaking although the carb needs to be fixed.
In other words...
Find if it is leaking enough to see the leak in the floor (cardboard).
If it is leaking and you cant find it by looking at the cardboard then pull the carb out of the boot and put something that can catch the gas if the carb is leaking (chemical resistant glove, plastic container, etc....). If you still loose a gallon overnight and see no sign I will work my way up. My money is on the carb overflowing... Check the pull start rope for oil in it.
Good luck.