According to Wikipedia, Knight Rider 'jumped the shark' in the very first series! On its list of different ways a programme has gone too far, it says:
I could understand the Michael/Garthe episodes more in season 2 but the programme was very much in its prime.Use of a plot device which is regarded as a cliché, such as a story involving the evil twin of a main character; e.g., on Knight Rider, KITT's meeting his bad-minded brother KARR.
If anything, looking back, i'd say the introduction of RCIII heralded the end of Knight Rider more.