But, if you have a Victron DC-DC charger this unit (which model/year??) should be able to know if the truck is running or not.
And this has to be the only way in for current to the Camper.
As you say your camper is from 2003 there might be wiring issues that take shortcuts around your B2B charger.
I've seen to many old campers with horrible hacked wiring in the last couple years on campgrounds with owners desperately looking for help to get it fixed.
Everything is just cheap cheap cheap, lousy assembled and then hacked by some shade tree DiY electricans.