Grounding house battery to vehicle chassis
possibly something like this.







simulate this circuit &ndash; Schematic created using CircuitLab

the battery interconnection forms a diamond shape so that each battery sees the same load and charge voltage,  the system is grounded so that the alternator can charge the batteries and so that low power 12V loads can use the chassis as return.  a switch is fitted so that discharging of the engine battery can be prevented.