Crabber50 -- Just out of curiosity, what gauge wiring did Jayco use for your trailer? If I understand the setup correctly, Jayco designers put the power panel at the rear of the trailer -- about 30ft downstream from the battery bank -- and attached to the power panel or located nearby is the converter. Is that correct? If so, I would love to know the gauge wiring used.
My thoughts about your electrical project: yes, you can run romex wiring from your power panel up to a junction box in the vicinity the new converter (unlike the link you provided that gave you the idea of an additional converter, I would not cut the pigtail of the new converter and wire to the romex. why? by doing so you'll void your converter warranty). Assuming that there is an empty circuit breaker in your power panel, you could install a new 15amp breaker that would be dedicated solely for the new converter. This assumes that you have easy access to the power panel, in which case, you would simply flip the breaker "on" when you want to charge the batteries, and "off" when charging is not needed.
With respect to the wiring for your DC voltage circuit: yes, you can use 4 AWG wiring for the new converter circuit -- and as I understand it, you're looking at a circuit of 12ft in length (6ft each way). The positive wire should be connected directly to the battery post. You can run the negative wire to (1) trailer frame assuming that there is another negative wire from the frame to the negative post; or (2) run the negative wiring directly to the negative post.
I also recommend that you install a fuse in the positive wire for the new converter -- as close as possible to the positive battery post. Normally, the fuse size is based upon the wire gauge. In this case, a 125 amp fuse is properly sized with 4AWG wiring. However, since you're only using the 4 AWG to minimize your voltage drop, where you're only going to be using a 55-70amp charger, you could use a smaller fuse such as a 80amp fuse. Using a smaller fuse than what is recommended (ie, fuse sized to wire size) is acceptable. Remember, the fuse is used to protect the wiring.
Hope this helps.
2004 Suburban 2500 LT 8.1L/4.10/4x4
Putnam XDR Class V, Equalizer 1400#
2006 Jayco Eagle 264BHS
Iota DLS-55, Progressive Industries EMS-HW30C, Trimetric TM-2025