Maybe my story will help in trouble shooting your issue.
10 years ago I owned an '01? Alpenlite 5th wheel. Having slide motor problems I replaced the old motor which was attached to the gear box. I got the new unit (motor&gearbox) from Western RV in Yakama, Washington. After replacement I opened the old gear box to find a loose washer jammed between 2 gears, which was the reason the motor did not work. Removing the washer allow the entire unit to work properly. The gear box was made by H&H Engineering in Battle Creek, MI.
'04 Newmar MADP, 1100w of solar, towing a jeep wrangler