Have the Feds spent those funds elsewhere, instead, such as paying the Obamacare “navigators” to scam people into signing up or a failed program?
You have it backwards.
Fuel taxes don't provide enough money for funding and maintaining highways. Federal and state income taxes help make up part of the difference.
Something else to keep in mind is that fuel tax revenues are falling over time -- due to the effective decline associated with inflation and the actual decline brought about by improvements in fuel efficiency. We buy less fuel for every mile we travel today than we did ten years ago.