To increase the life of slewing reducers, the reliability of their gearboxes is essential. They should be designed, simulated, and tested considering the installation environment and actual load conditions. Many studies have been conducted on the reliability improvement of gearboxes for reducers; however, a few studies have considered actual input load conditions. Furthermore, there is insufficient research on the effects of housing stiffness on gear misalignment, gear life, and load distribution and maximum contact load of the gear tooth flank. In this study, the effects of housing stiffness were analyzed using input torque measurements to investigate the causes of ring gear damage during durability tests on a slewing reducer. The contact pattern of the gear tooth flank was analyzed by applying dyes to all gears inside the reducer, and the results were compared with simulation results. A large edge contact in the first-stage ring gear of conventional slewing reducers was suggested as the cause of the ring gear damage. A new simulation model using a gearbox housing with an increased stiffness showed that edge contact was improved by the decreased gear misalignment. No abnormal wear and/or partial fracture of the ring gear occurred during the durability test on the improved model under the same conditions as those of the initial model. These results indicate that an increase in the stiffness of the housing reduced the gear misalignment, improved the load distribution, decreased the maximum load of the tooth flank, and increased the gear life.