You have to access your water heater from inside the trailer. Look for a panel with a screw at each corner. It will be in the same location of the tt as the outside access. Mine is near the floor. There will handles on the lines near the heater. Typically perpendicular to the line is closed and parallel to the line is open. Turn the valves and fill the heater. Make sure water is in heater using overflow valve before turning the heater on. Check out the winterizing Winterizing forum
for lots of threads on the issue.