They refer to connecting to buses for the purpose of minimizing wire connections at the batteries. The only wires that should be hooked up to batteries are main battery cables to feed buses, branch circuits, engines, etc. and in the case of this charger, a temp probe.
The chassis ground is often connected to the engine block simply for convenience but a bonding system bus bar is fine too if easier to get at as the two are electrically connected. You just need to ensure the wire size is as stated in the installation manual.
The chassis ground is a more recent development since the original install. It is there in case there is a DC to chassis fault. It's done this way because of the high DC currents involved which an AC ground can't handle safely enough to protect the circuit. If you don't wire the chassis, your breaker won't trip in a DC to chassis fault. If there is an AC to chassis fault, the AC ground wire will easily handle the fault.