Where should I connect negative of smart charger

It’s impossible to say without knowing your exact setup Markbox, but generally the negative terminal of the battery will do the trick. If your battery is tied to chassis ground then you could connect to that and charge your battery no problem. But if you have any kind of battery monitor in place that uses a shunt resistor, then you would want to connect your charger to the chassis and not the battery negative terminal. In that scenario, it’s not that your battery wouldn’t charge, it’s just that your monitor wouldn’t register the incoming charge current and would not sync correctly.

Well you want to connect your charger negative to the terminal of the Victron Shunt that faces away from the battery negative, i.e. to the shunt terminal that does not connect to your battery. Otherwise your Victron will not see the charge. And connect all the load grounds to that point too. Or if like me you have your shunt connected to chassis at that point, then all loads and chargers can use the chassis ground.