these trucks use 2 each, 6 volt batteries to drive in slow, the put the other 2 6 volt batteries in series to get 24 volts for high speed.
If you wanted ME to start to try and fix this (not so young, and not so recent technology design*) truck, I would have you -start- by getting 4 known good 6 volt batteries.
also there is a resistor in this truck located behind the contactor panel's middle. you will want to check it. all the micro switches and contacts (and micro-switches on the contacts) would be the next step after the batteries.

* and there are plenty out there, still working away, that I wind up taking care of

My real suggestion would be to consider moving into this century, and consider a unit that will use less energy overall, one with transistors in the drive circuit. every brand has moved into this less expensive to own,use, produce technology and 'obsoleted' these older style units.