the motor controller should be mounted in the same place/area that MIB controller is.
it would be a larger square box with a blinking light on it, there might be more than one, it would also have some harness connections and larger power cables connected to it. Usually there is one for drive, hydraulic and in some cases power steering. I believe in this truck there are probably only drive and hydraulic. In most cases the drive controller would be mounted on the RH side and hydraulic controller on the LH side of the drive controller.
If you do not see anything like that there then remove the panel behind the operator and it may be mounted there. Most reach trucks have the motor controllers mounted in the same area, some do have components mounted behind a panel behind the operator.
If you don't have a handset or laptop program to access this controller then yes, you will need to call a tech that does have these tools.

Where would the Motor controller be located? I am not a technician but I do have a monster Toolset! Is this something I need to hire a technician to do? I have talked to a couple technicians and they keep telling me I just need to buy new batteries which is not an affordable option for me. I've also been told to go do a series of voltage tests while it is under load etc. etc. Your way sounds a lot easier.

no do not try and access anything on that controller with the ABCD buttons, that is the MIB (main interface board), all that controls is the custom settings for some basic drive settings and extensive adjustments for hydraulic functions and pre-programmed settings related to operator modes for drive/hydraulic functions.

The BDI settings will be maintained in the motor controller

Thanks so much ! How do I do that? I found a digital display with an A, B, C, D Button's - is this what I adjust?

you probably need to change the BDI reset voltage, lower it slightly because it may be set too high. When batteries get age on them they tend to not charge up quite as well to the high levels new ones can achieve. It sounds like its right on the borderline of resetting once it's finished charging.
The older curtis BDI's were like this and we were always adjusting the reset voltage to compensate for older batteries.