As Turbs already said, did you check to make sure water is in the tank by opening the pressure relief valve?
Let’s get the issue of the red switch that says tank heater out of the way. That switch turns on the 12 volt heating pads on your holding tanks and has nothing to do with the water heater. The purpose is to help keep your holding tanks from freezing in cold temperature.
Since this is your maiden voyage there is a high probability that you don’t have water in the water heater. If there isn’t any water in the tank and you turned on/off switch on the outside of the water heater you have probably burned out the element and it will have to be replaced. This was the electric option for heating water.
You can still get hot water using the propane option of the water heater. That would be the black switch with the word water above it and heater below it. Turn on the propane to your trailer by opening the valve on your propane tank. Purge the air out of the propane gas line by lighting a burner on the stove. If this is the first time you’ve used the stove it might take a minute to purge the air. When I do this I turn on the stove burner and then hold a flame to it until it lights.
Once you have propane in the line turn on the water heater switch. After a couple of seconds the DSI/FLT light will come on and stay on while the DSI tries to light the burner on the water heater. Once the water heater ignites the DSI/FLT should go out. If there is water in the water heater you’ll have hot water in about 15 minutes.