Chiri, Mr C is somewhat right and Cobraboy is correct in pointing out that empty tanks tend to create more condensation. I've always tried to keep my tanks full,...but ONLY in my boats kept in the water where the marine environment contributes to condensation forming in the tanks.
I routinely leave my current boat, a Shamrock, gas-powered inboard that is stored out of the water on a trailer and my Toyota Tundra truck kept in storage in Florida for up to 6 months at a time. I also leave them both with the tanks as EMPTY as possible as when I return, as I want fresh gas to run in them, not gas that has lost it's punch with age and can cause damage to today's fuel-injection systems. And with the crummy quality of the gas we recieve today, this is a REAL problem. If you leave the tank full, when you return you've got a FULL tank of crummy gas to run out unless you drain the tank. Even fuel stabilizers won't fully compensate for this deterioration. And the amount of moisture accumulating in an automotive tank that is relatively SEALED in today's vehicles is minimal and can be easily handled by the replaceable in-line fuel filter.
Another important contribution to condensation build-up is wide temperature variations such as those experienced in the northern US or Canada. Here in the DR, we DON'T have that kind of variation and internal tank condensation in a vehicle isn't as pronounced.
Leave it as empty as possible.