If the slide consistently does not close evenly on one side, it may be out of adjustment, causing one side of the slide to do all the pulling, thus the strained sound on the motor. I suggest looking for an adjustment point at the end of the slide arms.
I had a Dutchmen years ago that used a solid axle with two pinion gears that ran in a rack on each slide arm. One of these pinion gear's set screw came loose and the pinion gear jumped off the rack, causing the problem you describe. I squared up the slide, raised it slightly, slid the pinion gear back in place, and it was fine. That trailer had hydraulic slide. Good luck.