OK, let's start by saying no uses one battery to charge another.
Yes the physical connections are often attached within the source circuit directly at the terminals of a battery on that side, for good reason.
But, the actual **source** of the charging energy is the alternator, a mains charger, a solar setup etc.
The connecting device (e.g. DCDC charger, but not necessarily) only closes (combines) the two circuits when a source like that is active, usually by a direct external switch (solenoid / relay / contactor opening, electronic signal) or by an internal circuit with VSR/ACR functionality.
OK.
So, I take it you want whatever source(s) keep(s) your power pack (battery) in your bed charged,
(your stock truck alternator?)
to also charge the House bank in your trailer?
There is no reason for the power pack to participate in this, since the more robust and cheaper Starter batt is already available to act as a buffer.
You need a robust and very thick gauge wiring infrastructure from the alt circuit, perhaps direct from the Starter posts, all the way back to the trailer House bank.
Likely you will need / want a DCDC there to overcome the resulting voltage drop.
Andersons do make good connectors.
If the connection from the starter circuit to the power pack is **very** robust and thick gauge wiring, it would be possible to charge both target banks at the same time, but I'd actually check the continuity, resistance and voltage drop of all the components carefully, at every point along the way.
