  RX~J1713.7$-$3946 is a prototype in the $\gamma$-ray-bright supernova
remnants (SNRs) and is in continuing debates on its hadronic versus leptonic
origin of the $\gamma$-ray emission. We explore the role played by the
diffusive relativistic protons that escape from the SNR shock wave in the
$\gamma$-ray emission, apart from the high-energy particles' emission from the
inside of the SNR. In the scenario that the SNR shock propagates in a clumpy
molecular cavity, we consider that the$\gamma$-ray emission from the inside of
the SNR may arise either from the inverse Compton scattering or from the
interaction between the trapped energetic protons and the shocked clumps. The
dominant origin between them depends on the electron-to-proton number ratio.
The diffusive protons that escaped from the shock wave during the expansion
history can provide an outer hadronic $\gamma$-ray component by bombarding the
surrounding dense matter. The broadband spectrum can be well explained by this
two-zone model, in which the $\gamma$-ray emission from the inside governs the
TeV band, while the outer emission component substantially contributes to the
GeV $\gamma$-rays. The two-zone model can also explain the TeV $\gamma$-ray
radial brightness profile that significantly stretches beyond the nonthermal
X-ray-emitting region. In the calculation, we present a simplified algorithm
for Li & Chen's (2010) "accumulative diffusion" model for escaping protons and
apply the Markov Chain Monte Carlo method to constrain the physical parameters.
