We present a new Monte Carlo method to simulate ionic liquids in slab geometry at constant potential. The algorithm is built upon two previous methods while retaining the advantages of each of them. The method is tested against a Poisson-Boltzmann theory and the constant surface charge ensemble, achieving consistency among all of them. We then analyze the computational time of the developed algorithm, showing substantial speedup in relation to the method of Kiyohara and Asaka [J. Chem. Phys., 2007, 126, 214704]. As an application of our method, we investigate crowding and overscreening in confined room-temperature ionic liquids. We show that we can switch between two behaviors of the double layer by changing the Bjerrum length alone.