Nobody knows where the two sets of two wires come from or go to, or what size they are. A 1000watt inverter will draw around 100amps and it's unlikely that the existing switch or cables are suitable - and is the battery big enough? Assuming the leisure battery is somewhere in the back of the bus and the switch is at the front there will be a voltage drop in the cables and the lower the voltage the more amps the inverter will draw.
Without knowing details of the existing setup it's obviously impossible to advise but you'll probably have to run new cables from the leisure battery to the inverter through a fuse or circuit breaker, the cables must be rated for the current and kept as short as possible to reduce voltage drop.
Without knowing details of the existing setup it's obviously impossible to advise but you'll probably have to run new cables from the leisure battery to the inverter through a fuse or circuit breaker, the cables must be rated for the current and kept as short as possible to reduce voltage drop.
