Well first let's determine if it is 12v
or 115v. See if there is any voltage there with your multimeter.
Next, take a look at the 85 Excella center bath floor plan on Airstream.com. Try to determine what might have been installed over the water heater.
My 86 has numerous 12v
"devices" like reading lights, under cabinet lights, thermostats, and the like. These are 12v
wires penetrating the interior skins.
Other Excella owners may chime in and know exactly what it is based on your photo.