First, I understand that most (all?) FEMA trailers were built for fixed location use, so they might not include normal RV equipment.
There are 2 brands of RV water heaters, Suburban and Atwood. All RV water heaters are LP gas operated and some models also work on electricity (dual use). Most newer RV water heaters have electronic ignition for LP gas operation. These are turned on from inside the RV. Usually there is a switch with a light on a wall in the kitchen area. You turn the switch on, the light comes on and then it goes off as soon as the WH ignites. If the WH does not start the light stays on. To use the WH with electricity there is normally a switch inside the WH which is accessible when you open the outside access cover. For electric use there is also a water heater breaker in the electric panel. I found the owners manual for the Atwood WH on the internet:
http://www.atwoodmobile.com/manuals/wat ... 4.9.09.pdf
The Atwood is very similar to the Suburban.
For the Suburban WH manual you have to call them: (423) 775-2131
I hope this helps! Good Luck!