Differential conductance of the resonant-tunneling structure with a single impurity level studied in the current plateau regime undergoes fluctuations around a zero average manifesting the energy dependence of the local density of states in a disordered electrode. Although the rms value of dI/dV depends on disorder and barrier transparencies, it is almost independent of temperature and, as a function of bias voltage, has a correlation function scaled by the intrinsic width of the resonance, which can be regarded as a tool to measure this quantity beyond the main differential-conductance peak. The analysis is extended to the regime of classically high magnetic fields where both the amplitude and correlation magnetic field are expected to increase.