Steel fuel tanks are generally bare metal inside. The "air" inside becomes almost entirely fuel vapor fairly quickly, so there's very little oxygen to cause rust. Any water vapor gets dissolved in the fuel - if too much of that happens the water separates and pools at the bottom of the tank. This is why old tanks are often rusted at the bottom where that water pools.
The bare metal should not rust in most cases on a regularly ridden bike where the fuel is refreshed regularly. In very humid environments you should regularly add some gas line antifreeze (methyl alcohol) which absorbs water from the fuel or pool, and it gets passed through the engine.
Parking a bike for a long time can be trouble if the tank is not full: air will be breathing in and out as the air pressure changes with weather and tank temperature and the new air coming in will have some moisture which will accumulate. An empty tank will collect water this way as well. An empty tank should be stored in a dry environment with the cap open.
