I would also be looking at where the wires to the slide out flex often.
You might try running a new ground wire between light #1 and #2, to see if that might be the problem. Make sure you are attaching ground to ground, or fuse #15 will blow in addition to the fuse that runs the other light.
Normally they run multible fuses for the lights, so that if one blows, you will still have lights in part of the RV.
Also the slide out wiring is probably connected in the basement, while other roof mounted lights are connected overhead.
Chances are that where the wires flex, there is a loose connection, perhaps a loose wire nut, or a flexible wire that has flexed one to many times, and nearly all the copper wire in it is broken, leaving just one strand to carry the 7.6 volts under load.
However I am also wondering why when you are running a 0.02 amp LED light and a 1.2 amp normal bulb, you might still get 7.6 volts to the open side of that light. LED's normally use well under 0.5 amps, and normally there has to be a 'huge' voltage loss or very weak connection to cause the voltage to drop at all!
Fred.