There is ususally a six gallon water heater. It can be propane or propane/electric. Some have pilot lights, some are electronic ignition. On a '86 it probably has a pilot light. They can be located just about anywhere in the trailer, the easiest way to find it is to look for a door like the middle one on the back in this picture:
Once you find that door, or one similar, get the model # and we can give you more specifics on how to use it. Also, look in the files section, there might be an owner's manual.
2004 Dodge Ram 2500 Club Cab 4x4 CTD
2007 Sunline Solaris T-276-SR: Misty Blue/Cherry
2001 Dodge Ram 1500 Reg Cab 4x4 OFF-ROAD