I don't think I have ever seen a 50A(F) to 15A(M) adapter.
What I have seen some guys do is use two of their normal adapters they carry to adapt down to the 120VAC 15/20A RECEPTACLE...
This is a drawing I found on google showing your two 50AMP ZONES being adapted down to a 30AMP Adapter source.

You will just a second adapter in front of the the 30AMP Adapter shown above...
You need to have these two adapters to do this...
50AMP(F) to 30AMP(M)

NOTE: this adapter will connect to both of your 120VAC 50AMP ZONES in your trailer but will only provide up to 30AMP for both ZONES. The 30AMP breaker providing this power if you are plugged into a 30AMP SOURCE RECEPTACLE will trip if you exceed 30AMPs being drawn by the trailer.
To get down to a 15AMP 120VAC SOURCE then you plug into this adapter a second adapter 30AMP(F) to 15AMP(M) Adapter to be able to plug this whole contraption into a household 120VAC 15AMP Receptacle
NOTE: Now you will only be able to draw a total of just 15AMPS from BOTH of your 50AMP 120VAC ZONES in your trailer. If you draw more than 15AMPS of current the circuit breaker of the household 120VAC Receptacle will trip.
You are safe as no MALE PLUGS you may have out in the open will never have HOT AC VOLTAGE present on them. Only the FEMALE plugs will have HOT AC VOLTAGES present when plugged into a power source..
All of this sounds like an accident waiting to happen but some folks need to do this I guess...
Have fun
Roy Ken