I just purchased this charger. All my research indicates that it should be mounted as close as possible to the coach batteries that it will be charging. It must therefore be mounted in the trailer, not the tow vehicle. As the victron Orion senses the voltage of the alternator and uses that voltage level as an “on/off switch”, a secondary switch is unnecessary but the charger can be configured with a manual switch if you desire (check out the wiring diagrams on Victrons website for more detail.
The Orion 12 12 30 charger is available as an “isolated” or a “non-isolated.” The difference is that the isolated had a negative input terminal and a separate negative output terminal. The non isolated has one negative terminal that would attach to a common negative ground (busbar). The isolated model has circuitry that isolates the alternator “noise” from the charge output. This is handy in a marine application where VHF radio performance may be obscured by the “alternator whine.” I can’t see how this would be an issue with RVs so I bought the “non isolated”
Once I have mine installed, I will post pictures. In the meantime, you can see a few installs on YouTube. Good luck!