Since there's been no other replies, I'll submit that changing/modifying the dome light in the cab might be a less preferable option to adding a second light to the dome light circuit.
An additional surface mount LED light added to the existing light circuit would be very easy to mount, draw very little extra current on that circuit - and would operate the same as the regular dome light. This gives options for 2-3 light levels as you desire.
A reason that a very bright dome light with no low-light option is bad, is that when entering and exiting the cab at night, a very bright dome light will blind the driver
Our dome light has 2 light levels to select from and we have found it very useful...like:
https://www.amazon.com/Kohree-Ceilin.../dp/B01L8CW8Q2
Best luck