TT - did you ever verify the 120Vac breakers that the Power Line Monitor is attached to (or it's electrical connections, since it basically plugs into 2 outlets)? See my response to your question about this last year:
FT Power Monitor (http://www.foreforums.com/index.php?topic=23208.msg180446#msg180446)
FYI I just verified that if I shut off our breaker that is for 1 leg of the Line Monitor and the block heater (the only things on that circuit/breaker) the readout for that leg on the Power Line Monitor says "01".
If there is nothing else on 120VAC in your coach (i.e. both air conditioners run fine is a good test to verify both legs of 50 Amp have power) that isn't working, check to see if one leg of your Power Line Monitor is on the engine preheat breaker and check the function of that breaker. We had that one (and others) get weak over time and had to replace them (readily available at Lowes, BTW). We discovered this after our Line Monitor would show one line at "01" after driving and we would find the breaker needing to be switched fully off then on again to reset it.
If it is not the breaker, check to see that both plugs of the Line Monitor are plugged into outlets.
Michelle
Update and results of a forensic investigation of my Power Line Monitor.
My Line 1 has always displayed "1" since I purchased Ducky on 8/8/2014. Line 2 normally shows about "125". Recently, it has started jumping around, so I decided to open it up and see what makes it tick. These are my findings.
After removing the PLM faceplate, we see the main circuit board (inside a metal box):

The primary inputs to the circuit board are attached on the seven screw terminals at the top of the circuit board:

For convenience, I have named them from left to right as 1-7. The actual connections are labeled on the circuit board as follows:
1 = LINE2 (black wire from line 2 cable)
2 = NEU2 (white wire from line 2 cable)
3 = GND2 (green wire from line 2 cable)
4 = BOX GND (green wire to screw on box)
5 = GND1 (green wire from line 1 cable)
6 = NEU1 (white wire from line 1 cable)
7 = LINE1 (black wire from line 2 cable)
I took voltage measurement among the different screw terminals with the following results:
1-2 = 119.4
1-3 = 119.4
1-4 = 119.5
1-5 = 119.6
1-7 = 240
5-7 = 119.7
6-7 = 119.8
Obviously, both Line 1 and Line 2 voltages were getting to the main circuit board. After removing the circuit board from the box, I discovered signs of a burn on the top right of the box, under the Line 1 area of the input:

Whatever caused those burn marks did even more damage to the bottom of the circuit board:

That certainly explains why I never got a reading for Line 1 voltage on the PLM output display for line 1.
The display circuit board did not appear to have any problems on the top side:

But the backside also had a slight burn mark at the edge:

True confession time: that mark was NOT there when I was taking my voltage readings. After the readings, I moved the box to the side and heard that ZAP, POW sound, along with a flash and that great electrical burn odor. Do you know how long it takes to go from standing there to flipping all three of the circuit breakers on the two power panels? I don't either, but I think I may have set an Olympic record.
The next questions is, "What now?"
More about that
tomorrow later,
Trent
ETA: I tried to embed the images, but it did not work. With Michelle's help, I think I got it now! Thank you, Michelle (and Steve).