checking the voltage drop during driving is both dangerous and does not show anything worth reading*. you must check the voltage drop when the hydraulics are fully extended and 'stalled', or use a load bank built for this.
you need a [high amps] load against the motor to load [high amps] the battery

* traction motor currents change from being a driving force (a motor) to being a driven force (generation of current) and currents like flyback and regen currents switch at way to fast a rate to make reading with a meter to not be of any value at all, imnsho.
As this unit has a lift controller, and you seem to have proper signal from the hydraulic valve switch/pots to the controller, I would want to confirm the input is on the proper place and then go into checking the controller, setup and operation.