Tons of King of the Hill fan theories exist throughout the Internet, from ones alleging Jeff Boomhauer is an undercover spy to others speculating Peggy suffered a serious brain injury, and an infamous theory that Hank isn't Bobby's father. But none quite hold a candle to Dale Gribble King of the Hill fan theories. When you think about it, the endlessly suspicious Dale makes for excellent speculation, his own neuroses only emphasizing the character as a victim of conspiracy. Dale Gribble knew Joseph wasn't his son, literally so in an episode after realizing he was out of town during his son's conception. Although, to be fair, he believes Nancy got impregnated by aliens.
But Dale knew about Nancy's affair too. With Dale monitoring all incoming calls, Nancy's talks with John Redcorn definitely failed to go unnoticed. Plus, he literally catches Redcorn sneaking into his bedroom with Nancy's help in the middle of the night. If Dale lives such a paranoid conspiracy theorist, why hasn't he figured out that his wife has been cheating on him for 14 years? According to fan theories on Dale Gribble in King of the Hill, he has figured it out, and the proof lies in his story.
A subtle throw-away joke appears when Dale claims he "monitors all incoming calls." Okay, that makes a lot of sense because Dale clearly suffers Paranoid Personalty Disorder. But if he listens in on all of Nancy's phone calls, he likely also heard conversations between her and her "secret" lover John Redcorn.
Seriously, just look at them. Joseph looks nothing like Dale, a scrawny, white guy who reads romance novels. But still, Dale remains extremely proud and supportive of Joseph's athletic prowess, because Dale possesses none of his own. John Redcorn, on the other hand, sports the same skin tone, eye color, hair color, jawline, and overall personality traits as Joseph.
In an early episode, Dale walks in on Redcorn sneaking into his house through a window. Assuming he's been caught red-handed, Redcorn tells Dale "he regrets him finding out in this way", but Dale gives absolutely no reaction. Later on, Redcorn even mentions Dale's complete and intentional obliviousness to the situation "really takes the fun out of it."
Dale has it easy. His wife Nancy takes care of the bills because of her high-paying job at the Television studio, which leaves him free to run his exterminator business and drink beer in the alley with the guys. Plus, he b a great relationship with Joseph, and getting a divorce would ruin that.