This is probably complete overkill, but I have insomnia and nothing else to do. I've seen the movie on the western channel.
If you want to use the above pic of the adapter cord as a roadmap, here is how to test to see if you have something wrong with it, utilizing a multimeter.
With the adapter cord plugged into a 120 volt power source:
Set you multimeter to measure AC voltage (120 volts or higher). Make sure not to touch the test probes with your hands since you are checking live voltage. Keep your fingers back on the rear plastic part of the probes.
Place the black probe into the neutral of the NEMA SS2-50R receptacle. This is the one with the W on the pic.
Now place the red probe into the L1 which is the X in the pic. Your multimeter should register between 110 and 125 volts usually.
Leaving the black probe in place, now put the red probe into the L2 which is the Y in the pic. Here again you should have the same voltage reading as you did when probing X.
If you don't have voltage either in the X or the Y, then you have a faulty adapter....and thus are not getting power to both your L1 and L2 inside your RV.
With the adapter cord unplugged from power source:
Set your multimeter to Ohms (aka resistance or continuity)
Put one probe on the L1 male terminal of the NEMA 5-15 P. Now place the other probe into the X on the NEMA SS2-50R.
Leaving the probe on the L1 male terminal, probe the Y on the NEMA SS2-50R.
If you don't have the same reading for both X and Y, here again it's probably a faulty adapter.