Blazars are powerful sources of high-energy (HE, $\gtrsim 100$ MeV) to very high-energy (VHE, $\gtrsim 100$ GeV) $\gamma$-ray emissions. According to the unified scheme for radio-loud active galactic nuclei (AGNs) radio galaxies are the parent class of the blazar population. Naturally one would expect radio galaxies, as the parent population of blazars, to be intrinsic HE and possibly VHE $\gamma$-ray emitters. This was confirmed by recent VERITAS, MAGIC and HESS detections. Previous works found that secondary electron/positron pairs can become efficiently isotropized by mild magnetic fields in AGN environments, leading to off-axis HE–VHE $\gamma$-ray cascades. We extend this work by including a more realistic Shakura-Sunyaev (SS) accretion disk model, in addition to the isotropic external radiation fields considered in previous works. We present the preliminary results of the interplay between two external radiation fields, namely the broad-line region and UV emission from the SS accretion disk, and then discuss future developments for this work.