The frost spot will occur most likely at the vapor/liquid interface where evaporation is taking place inside the vessel.  The pressure inside the vessel will determine the saturation temperature and ice will only form when the saturation temperature is significantly below 32°F, especially if there is 5 inches of perlite insulation.  If the insulation has gotten wet before then it can be worse than no insulation at all and the frost spots may persist where the insulation has been damaged/wet.  Solar radiation effects must be taken into account when determining the energy balance for the tank.