Water heater lighting

I'm pretty sure the previous owner said it was propane water heater

depends if it is DSI or manual, one way to tell is open the door, if there is a big blue knob ( the thermostat)on the front,it is manual light and you will need to turn a knob on the top of the area above the burner to pilot, hold it down and light the pilot hold for a few minutes and let the button up, then turn it to heat
either way make sure there is water in the heater and air is out of the lines also run the stove for a few minutes to get the air out of the LP line.

If it's a FEMA trailer, I don't think they use gas.

There should be a switch somewhere to turn it on. then if you have battery coming to it and gas it should light right up. sometimes a second switch on the heater itself.