If the motor move the slide ok to that point, I certainly wouldn.t think you need a new motor. Does the slide/motor just stop, or does it lug down to a stop? If it just stops, I'd be tempted to hot wire the motor, see it it completes its travel. If it does, I'd then look for a limit switch, and adjust it. If it lugs to a stop, find out whats stopping it,....like adjustable stops ?? I doubt it has been that way from day 1 ??
You can go here http://www.powergearus.com/fleetwood-tech-library and try to find documentation for your slide. Mine has an adjustment for how far it goes out, undocumented of course, but the exploded parts list helped me track it down. If yours has an adjustment there's not a chance in the world that it's the same as with my kiddie car motorhome.
There is no adjustment on it. It suppose to stop when the motor hits a certain voltage I believe. If that's the case, does that mean I need a new motor