By inverter, I suspect you mean the power converter, the master power center? When speaking in RV terms, converter usually is used in reference to the power center that brings in 120v AC power, distributes 120v circuits inside the PUP, converts the 120v AC to 12v DC and is also the distribution panel for those circuits and last but not least, has a connection to the battery for charging it, if so equipped.
An inverter is typically referred to as an add-on inverter that takes 12v DC and changes it to 120v AC to power things such as televisions, DVD players and the like.
As for tapping into an existing 12v DC line, most of them IMO are under-utilized.
First, identify each of the 12v circuits and what is powered by them. I have added 12v lights to existing circuits after figuring out what is on the rest of the circuit and determining how load is needed if all are on at the same time.
The nice thing about doing this you don't have to run any wires to the outside of the PUP and have an oddball connection hanging off of the battery. You just get some automotive splice connectors,
and tap an existing line.
What's the draw of this junction box, # of amps?
You certainly could run a new line from the battery directly to it. Be sure to install an in-line fuse so you don't have a possible run-away situation.