“Yes, that is right. The power from the tow vehicle charge line comes in downstream of the battery isolation switch (at least on our trailer). Meaning when plugged into a properly wired tow vehicle it is live and power to the trailer is unaffected by the switch. Easy enough for owners to check on their trailer by ensuring no shore power, hooking up their 7 pin, turning on their tow vehicle and flipping the isolation switch. If any 12V lights stay on in the trailer then obviously the isolation switch is not killing the power from the tow vehicle charge line.“ (quote from Rubicon)
You might well be right for my trailer, too. The proof will be in the testing. It also means that the tow vehicle charge wire runs all the way back to the panel, then forward all the way to the batteries. On my little trailer this adds 25 feet of voltage drop before the tow vehicle charge reaches the batteries. No wonder people report that trying to run the fridge on 12v while driving drains the batteries.