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Wales captain Elliott Kear, Ant Walker, Ashton Robinson and Connor Davies also went over in a dominant first-half display.
Moxon saw Mike Butt and Huw Worthington go over as the rout continued.
Serbia battled to the end, but were thwarted in attempts to get on the scoreboard and saw Joe Coope-Franklin cross with the game’s final play for a debut try.
Wales’ tally against Serbia is now 3-0 with an aggregate points score of 186-8 to Wales.
Before Wales won in his absence, boss Kear criticised the qualification process for the 2026 World Cup.
In a statement Wales RL said Batley Bulldogs head coach Moxon would continue to be in charge of the team for Saturday’s encounter.
“All at Wales Rugby League send their best wishes to John, and ask everyone to respect the privacy of him and his family so he can focus on making a full recovery,” added Wales RL.