Starting from fifth, the Queenslander maintained the same position throughout the race, despite pressing Zaid Ashkanani for all of the 16 laps contested.
“I attempted to keep up the pressure, but Zaid put in an immaculate drive,” said Campbell.
“Considering it's my first Monaco race, I'm happy with fifth place.”
German Michael Ammermüller scored his third-straight race win in the series after taking a double-victory at Barcelona.
Second and third became Dan Cammish and Dennis Olsen, providing an all Lechner podium. All of the top cars finished in the positions they are started on the famous 3.340km street circuit.
Campbell sits fourth in the overall points standings and second behind Olsen in the Rookie title chase.