Start on the outside of your trayla: there will be a hatch about 16" x 18" that has about a quarter of its surface cut with vent slots. Careful they DO get !HOT! when the water heater is heating water!
Now just figure out where on the inside that unit has to be. They are almost always in or near the bathroom. Now, I know your trailer is big, but the water heater is pretty big, too, so it can't hide for long from you
The shutoff vales should be obvious: the bypass is just a short pipe that connects between the two pipes going in and out of the water heater's tank.
The handles are closed when they are at right angles to the pipe they are on. Similarly, they are open when they are in line with the pipe they are mounted on. Which only makes sense, when you think about it.