Victron did think of that. There is hysteresis between the 'on' and 'off' voltages as well as a smart mode, which momentarily disables the charge current to check and see what the truck battery/alternator voltage is without any current flowing and loss in the wire. For example, say your alternator puts out 13.7V and you have a 1V drop in the wiring - set the 'on' detection at 13.6V and the off detection at 12.5V. When the truck starts, the truck battery quickly goes up to 13.7V, but at that point there is no current flowing to the DC-DC charger, so it also sees 13.7V and turns on. Once it is on, there is 1V drop so the DC-DC is now seeing 12.7 on the input, which is still high enough to stay on. You stop the truck, the battery quickly drops to its resting voltage of 12.8V, which means the DC-DC is seeing 11.8V after the 1V drop which is below 12.5V, so it shuts down.
I am certainly not suggesting that there is anything wrong with running large wire from the alternator to the DC-DC charger, if you have done that, or it easy to do, then that is the ideal situation. In that case you may not need DC-DC charge as the house battery may get a better charge directly from the alternator.
But one of the main reasons to install a DC-DC charger is to compensate for voltage drop on the wiring to the camper, which is why you generally don't get a great charge to the house battery from the alternator with the stock wiring. Putting in a DC-DC charger is often cheaper and easier than re-wiring the truck and camper with larger wire. A 20A DC-DC converter will likely work on the stock 10AWG wire, and is within specifications. A 30A DC-DC would be marginal, and in that case you would probably want to at least upgrade the cable from the alternator -> camper, even if you don't want to disassemble the cabinets to get to the 10AWG inside the camper. This is what I have done, and while I am using a home-made DC-DC charger not an Orion, it works well.
Again, I am not trying to argue that you shouldn't use larger wire. All I am saying is that you may not need to use larger wiring - the DC-DC will compensate for the voltage drop in the wire and boost the voltage (and current) to the house battery.