Trump's 2020 re-election campaign recently filed their required FEC statements, showing they had raised $13.2 million so far. It's safe to assume at this time that Trump does, in fact, intend on running for a second term, like almost every other first-term President has. It's way too soon to tell if he will have a serious primary challenge (unlike most incumbent Presidents), but even if he does, he can clearly spend these campaign funds on that election.
However, what happens if Trump can't run for a second term? What happens to his campaign funds if he is impeached (and actually removed), resigns, or dies in office?