1981 Gas Tank Capacity

Can anyone tell me how to find out the capacity of the gas tank for an 81 V-20 Step Lift? I jsut brought one home this afternoon and need to winterize it before the snow flies and I want to add a fuel additive but need to figure out how much fuel it holds to get the right ratio. Other than taking the rig to the gas station and pumping till it runs out the vent, is there any way one knows how to tell the size of the tank? Thanks in advance...