Power?...are you sure?
You state fuses and connections are OK. Did you measure them with a voltmeter?
No meter? GET ONE for the MH they are cheap.
Since the horn works when you activate the switch but the lights in the switches do not work I suspect you have a BUSS problem.
THe switch is just a controlled broken wire, the wire goes through the switch and nothing much exciting there.
The lights however need a voltage potential to operate, positive 12 volts and a ground or return path.
Depending on how the MH was built and the monitor installed, you could have a buss tye install where a lot of wires are connected to a common point, then this point is connected to the source.
Common buss is ground, you have either a large terminal or a single wire daisy chained across the back of everything finally connecting to chassis somewhere.
Use your voltmeter and measure the voltage AT the monitor.
It should have both red and black wires, use shirt pins to puncture the wire, be carefull that they are not too close to short out.
Measure here, there should be 12 volts across them, if not, then measure each to ground.
If you have 12 volts on both, then the ground is not connected, 0 volts then the hot is not connected.
Take your time, and write notes and make a drawing as you go, it helps later when something else gets funny.