WIN *Get Windows Voltage
Remote control protocol manual:
<COMMAND WIN>
*Get Window Voltage
Controller --> Radio
WIN[\r]
Radio --> Controller
WIN,###,[FRQ][\r]     : A/D Value (0 - 255)
Returns current window voltage and its frequency.

However, the A/D value can be higher than 255, probably the ADC resolution is 10-bit.
uniden3500win-volt.jpguniden3500ver.jpg

u35-aten232uc1.jpg

Aten UC-232A USB-2-Serial converter works with Windows 10.