A full tank does not have a significant difference in the surface area of the fuel than it does at half, or even nearly empty.
There is always headspace above the fuel to ensure expansion/contraction room during temperature swings. On our unit, when we removed the 55 gallon tank to replace the pump, I found there was about 6" from the top of the tank to the top of the fuel when it is full. If the tank was filled completely to leave no headspace, it would probably take 70 gallons if filled via the fuel pump/gauge access port.
I keep ours full over the winter as an emergency resource in the event we lose power at the house.
Like others, I add marine Sta-bil to counter the effects of ethanol's attraction to collect moisture. The tank is, however, all stainless. There's nothing to rust but the moisture plays havoc with the engine and generator.
I don't believe there is any _significant_ difference to the fuel level as far as attracting moisture.
