Diesel will last years without degradation. The biggest issue is not moisture, but water in the diesel will allow algae to form in the diesel. This can clog filters, etc. Most common path of moisture is temperature changes which between expansion/contraction of the diesel and condensing of airborne moisture (dew) in the fluid. Using an additive can absorb the moisture and inhibit algae growth. I buy a product from the local HVAC supplier, made for heating oil tanks. A few ounces will treat my 50 gallon plastic barrels. I had steel barrels in the past, but then I saw the amount of rust that accumulated at the air interface and switched to plastic. You should not seal the barrels since there is thermal expansion/contraction of diesel and you need to have room for that to prevent overpressure. The best solution is to use a desiccant breather on the tank, but that is expensive and needs routine service on the desiccant (replacement or heating to remove moisture). I have a small oiled breather (like an oil air cleaner) on my tanks to reduce moisture ingress.
paul
paul