Central defender Moisander opened the scoring from close range midway through the first half for Ajax, who rested their entire forward line after Thursday's 2-0 Europa League win over Steaua Bucharest.
Dane Eriksen doubled the lead five minutes before the interval, finishing at the near post after a slalom run past three defenders.
RKC's Robert Braber missed the chance to get his team back in the match from the penalty spot as Ajax keeper Kenneth Vermeer stopped his attempt with his fourth consecutive penalty save.
"This happens often with keepers; if they stop two penalties they get confidence," Ajax manager Frank de Boer said.
"He kept us on track but on the other hand we should have sealed the win earlier as we had several chances."
PSV, 2-1 winners over Utrecht on Saturday, are top with 50 points after 23 matches, three ahead of Ajax in second.
Feyenoord suffered a shock 3-2 defeat at strugglers PEC Zwolle and slipped to fourth on 44 points, behind Twente Enschede, who were held to a 1-1 draw by bottom side Willem II Tilburg on Saturday, on goal difference.
An own goal from Graziano Pelle in the second minute opened the scoring for Zwolle before Darryl Lachman doubled their lead four minutes later.
Pelle pulled one back with a bicycle kick after 33 minutes and drew Feyenoord level with a close-range header a minute later.
Denni Avdic sealed the win, however, when he scored into an unguarded goal 15 minutes from time after Feyenoord keeper Erwin Mulder misjudged a corner.
Vitesse Arnhem remain fifth with 42 points after beating mid-table Groningen 2-0 thanks to second-half goals from Mike Havenaar and Wilfried Bony, who took his total to 19 with a penalty.
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