question

Fundamentally, this is no different than a standard car charging system, electric-wise.

First, since the batteries are in parallel, they will always be at the same voltage.

Second, while not consistent with general best practices, I recommend the inverter feed from the spare battery such that it takes the brunt of the load and will be charged a little less than the car's 12V.

The Prius AGM is high quality but relatively fragile due to its limited demand application.

I don't know the 2016 system specifically, but the 04-09 have a 70A DC-DC converter that provides charging and 12V power from the HV system - HV battery and . I expect the 2016 is highly similar. This means you really only have about 600-700W of sustainable output as the car consumes about 20A when running.

Victron does not make an isolator approved for use as a direct interrupt for an inverter. The surge associated with cap charging will blow one out once it reconnects the inverter. The only way a battery protect can be used is to signal a remote off switch on the inverter, i.e., the inverter must have this function - usually in the form of a relay or jumper that forces the unit off when a signal is received.

Ideally, you get a quality inverter that you can simply set the cut off to something like 11.9V. The Prius 12V does almost nothing during start up and only needs to supply 20-30A for a couple of seconds until the HV relays are closed and the HV battery/DC-DC converter are powering the car/starting the engine.

Something worth noting... the USABLE capacity of the HV battery (40-80% utilization) is almost identical to the TOTAL capacity of the 12V.

Lastly, get yourself a decent lithium jump starter for $50. Keep you from ever getting stranded.