I say, and with God's success: It is disliked to enter the restroom with the right foot, as the right side is meant to be honored; for he (peace be upon him) used to prefer starting with the right in all matters as much as he could. Among the honors of the right side is to begin with it in all good deeds, whether it be a hand or a foot, and to delay in all disliked actions. The restroom is a place of filth where the devil is present, due to the abandonment of the remembrance of God therein, so one should delay the right foot when entering and advance the left. See: Bada'i' al-Sanai', 5/126, and Tabyin al-Haqaiq, 1/166, and Fath al-Qadir, 1/419, and al-Tawdhih Sharh Muqaddimah Abi al-Layth, and God knows best.