I have a 2013 st31ss. Our water heater is under the panel beside the drivers side of the master bed. You can access it from the inside. Just removed the screws. If not, check outside for the hot water heater access panel and back track from there.
look along the wall for a panel with screws in it. it will be low near the floor. looking outside your trailer find the access to the hot water drain plug/overflow. (check to make sure plug is in tight). the inside panel you are looking for will be near that location inside.