No way in hell Hank Hill didn't vote for McCain. McCain was basically a nonfictionalized version of his father. And I'm about 90% sure he'd vote for Romney in 2012.
This, I there's a chance he would've voted third party or only very reluctantly vote for Trump in 2016. By 2020, he'd have either left the Presidential part of the ballot blank or very reluctantly vote Biden.
He definitely didn't vote for Hillary Clinton. As drug warrior as he was it's hard to imagine him voting for Gary Johnson, but I could imagine an episode with Special Guest Star Gary Johnson in which Gary pitches Hank on pride in a clean-burning joint with no added chemicals grown with American know how, a superior product to Mexican weed we should take pride in. Every time someone changes Hank's mind on something it's in that language. Episode probably also features Buck opening a dispensary but using low grade weed and Hank's professional pride overrides his hostility to drugs and he tries to set things right.
In all seriousness, Hank votes Kasich or Rubio in the primary. He considers Cruz but...no. when Trump wins the nomination, Trump's anti-Mexican rhetoric scares Enrique, and Seņora Peggy Hill is terrified no one will need lessons in Espaņol if Trump is elected, but Hank just can't vote for a Clinton. Everyone tells him Trump is unacceptable, but Hillary Clinton is Hillary Clinton. Hank looks at that touchscreen and either votes Trump or Johnson.
2020 is different. Hank would like Joe Biden. Probably the biggest hangup would be that Minh would be the biggest Biden booster on Rainey Street and Hank's always had issues with Khan and Minh. Minh would be insufferably loud because Ted Wasonasong's wife has been organizing and bankrolling the local TX Democratic Party and this is her chance to get in with the stars of the community. (Minh's dad is super right wing of course, and this is part of her rebel streak that led her to marry Khan at all)
