The problem i ran into adding a 120VAc Receptacle to my Trailer was I found the RV 120VAC Receptacles were the slimline push together type. This also pinches the 120VAC wiring to make connection.
I found hardly any slack in the romex and no good spot to tap into the 120VAC Circuit.
Google Image
I eventually found a spot with some slack and added a PVC Electrical Junction Box here. Then I ran a new ROMEX line from this tap to where i wanted the new 120VAC receptacles added.
I used the standard 120VAC Receptacle with screws on the back inside a PVC Electrical wall box...
Google image
One caution when working around the RV SLIM LINE TYPE 120VAC Receptacles is the ROMEX wiring has pinched the wiring lines and makes them very easy to break into because of the pinched crease in the wire. Due to lack of wiring slack it would be almost impossible to repair anything.
Just my observations
Roy Ken