You need to make sure your batteries are fully charged if your shore line is not plugged in.
Your slide has to come up a ramp and as things age we need a little more juice, or current in the motors case to push the slide up the ramp.
The control unit may have a pot adjustment that you can add more current to the motor.
If it does than only make small increments of a turn until it can move the slide in.
Make sure all your rollers are tracking correctly and not fallen off their holding brackets.
Ramp metal sheets should be in good condition and not loose on ramp incline.
Here is a
Newmar slide spec found in QT's 1 & 2 link.
You can see the pot on this slide control.
If you do not have this type than you
may find it in this link and if its faulty you can order from this link by double clicking on your type picture.
Let us know how you make out.