As the camper side is not connected to the VW and only receiving voltage through the split charge relay, it’s unlikely to be related to the engine swop etc unless the camper battery is stone dead and the split charge is not working.
OK a few things,
Are the camper lights working yes/no, no battery’s dead or disconnected, yes battery’s got some life, next battery has voltage check at battery min is 12v as heater and fridge cut off at 11v something...
No control display, engine running yes/no, yes battery dead, no no power to control unit, top fuse in camper fuse box behind drivers seat, if battery OK check fuse fitted and not blown, check voltage at fuse if OK check wiring to control unit.
You might want to check fuse first as a workshop that knows their stuff on campers would pull the fuse to stop the camper battery going flat.
Also have your tried hookup to shore power ie 220v