This paper describes modeling results for the on-orbit thermal response of spacecraft propellant used for orbit stationkeeping. The specific objectives of the work are to improve analytical models of a transient thermal Propellant Gauging System (PGS) in use on communications satellites. This simple method uses tank wall heaters and temperature sensors and a transient energy balance to obtain periodic measurements of remaining propellant fill. The fill levels must be related to the on-orbit sensor measurements through a thermal model of the system. In the past, relatively simplified representations have been used due to the complex liquid/gas geometry when in a weightless state. In the current work, advanced modeling tools are utilized to allow a more detailed representation of the actual liquid/gas geometry. Model results are presented for a range of fill levels later in life to assess PGS measurement resolution. Model results for a beginning of life case are compared to independent estimates of fill level to support model validation. Development of an accurate analytical model allows assessment of measurement accuracy and optimization of the hardware design and measurement procedure for different tank configurations.