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Robert, If I understand you correctly it sounds like your refer may run on either 12v or 120v, though it’s not clear whether it’s a compressor type (and if so, whether it’s a super efficient Danfoss type) or of an electric resistance heating element type?

However in general terms, the fact that you have the extra alternator makes it possible to charge the batteries while underway so long as the ‘continuous output’ doesn’t exceed say 25% of the alternators rating (in your case that would mean about 100amps max DC) to prevent overheating the alternator (gets mighty hot beneath the hood!)…

For a 12v resistance element type refer (meaning uber inefficient!!) of about 6 cu/ft capacity, this could mean an hourly draw of up to about 19amps (but only about a quarter of that IF a Danfoss!!), though the actual hourly duty cycle would typically be less than 1 hr.

Having mused that, this would require a dc-to-dc converter (and wire of a sufficient size, while considering length), but since your truck is near new you’d likely need a new generation type dc-to-dc converter. Generally speaking, newer vehicles have energy saving alternators that are programmed to only charge the batteries to about of 80% SOC (state of charge), then lower the voltage to save on fuel – These are called ‘Smart alternators’ so you’d might need (pending research…) a dc-to-dc converter that is designed specifically to work with the newer type ‘smart alternators’… Victron makes one and there may also be others…

At this point you’d have power when underway assuming that your batteries (which would be acting as a in-and-out reservoir…) can maintain a sufficient reserve while keeping up with the load, so the next consideration might be whether to use 12v or 120v via the inverter…Just saying from afar…