Roll-on/Roll-off (RoRo) ships are a crucial part of maritime transport to transfer cargoes between international regions. In this paper, we propose a novel mixed-integer programming formulation to stowage planning for RoRo vessels. Our model contributes to the state-of-the-art by modeling real-world stowage planning aspects such as multiple ports, multiple decks, optional cargo, and ship stability constraints. The objective of the model is the maximization of cargo revenues and minimization of the voyage costs.