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.
Mack
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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