Q: What happened to Dark Helmet, Colonel Sandurz and President Skroob? After they landed on the Planet of the Apes, what happened to Dark Helmet, Colonel Sandurz and President Skroob? 
Were they captured and forced into ape slavery? Was there ever a description of their ultimate fate?




A: They went on to further adventures in Spaceballs: the Animated Series.
Including opening casinos, playing video games and cloning dinosaurs.


A: We don't have enough information to come to any conclusions, but the apes themselves clearly assume that the Spaceballs will have the upper hand:

1ST APE
  Dear me, what are those things coming out of her nose?
The 2ND APE looks through binoculars. He sees COL SANDURZ on the ground, DARK HELMET is climbing down a rope, followed by SKROOB. SKROOB steps on DARK HELMET'S helmet.
DARK HELMET
  (mask up) Hey, hey, watch my helmet.
The 2ND APE stops looking in the binoculars.
2ND APE
  Spaceballs?
1ST APE
Oh shit, there goes the planet.

Being experienced bureaucrats, it seems likely that the Spaceballs will soon be destroying their new planet.

A: According to an interview with George Wyner (the actor who portrayed Colonel Sandurz), there was in fact some additional ending material that was filmed but never made it into the final cut:

Q. ...What do you think happened to Colonel Sandurz when he found
  himself on a planet of apes?
George Wyner: We actually filmed an ending that was never used. After the escape pod lands, and President Scroob, Dark Helmet, and
  Colonel Sandurz climb safely out, the three of them march down the
  beach together. President Scroob asks, "Where are we?" Colonel Sandurz
  answers, "California," and President Scroob barks out, "Let's pollute
  it!". 
Scroob, along with his trusted aides Dark Helmet and Colonel Sandurz,
  made a fortune polluting California, and is now considering running
  for president.

