Junction box question
In my motorhome I have a standard sized junction box (with cover) that has three 12/3 AWG romex runs in it. All three are wires are tied together - three grounds together, three white together and three black tied together with Wire nuts. All are 120V feeds.
I need to install another junction box adjacent to this one with an electrical outlet. I want to add a 12/3 nomex run from the existing junction box to the new junction box / outlet.
My question is can I splice a fourth 12/3 wire in the original junction box? Do I just need to use next size up wire nuts? There is room in the box. What is the proper way to splice 4 12/3 AWG wires? Should I even have four leads in a box at all?