We have analyzed a single-electron transport spectrum of a quantum dot formed in a GaAs/AlGaAs resonant tunneling device. The measured spectrum indicates a strong confining potential and the lack of circular symmetry. We model the quantum dot by applying a rounded parabolic potential and a negatively charged impurity, shifted laterally and vertically from the center of the dot. We analyze the physical conditions behind the optimized parameter values and consider also the structural properties of the corresponding single-electron states as a function of the magnetic field.