i tried to use normal modular rocket engines for rcs. assigning the pitch or yaw inputs works fine, but when setting the opposite side to inverted they dont function at all, even though a negative value on the slider should produce a positive throttle signal. (sorry that the engines dont align with the rcs nozzles, too lazy to fix that :P)
Enable "Show Advanced Properties "in the "Additional Settings" section of the part properties, now the Input Controller will have a lot more parameters to tweak, you will see "Min Value" is set to 0, set it to -1