Most common fault is probably a bulb that is outside the wattage range that has been programmed into the computer
Computer cuts power until the circuit is restored to the wattage programmed into it.
The amber side marker lamps are provided by the motorhome builder not Fiat and therfore are not part of the Fiat lamp monitoring system.
They are activated by a negative lights on signal that Fiat leave for the mh manufacturer to use. I believe Swift use a Sergeant system. There will be an interface, possibly somewhere in the driver seat area that provides the feed to the marker lamps.