1.) There is no statistically significant correlation between socioeconomic status and intelligence.
2.) Donald Trump's empire is built on his inheritance. There are reasonable arguments that his personal financial status has actually declined since receiving this inheritance.
3.) Donald Trump notoriously over-reports his own financial holdings to the media, typically coming up with ad-hoc estimates that have unrealistic padding for personal brand value (variously numbering in the billions.) Credible estimates range from $250 million to $3 billion. It is difficult to make an accurate assessment because almost all of Trump's capital is tied up in unsuccessful real estate projects. It is possible that even Trump does not understand how much he is worth.
4.) Donald Trump is, essentially, a scam artist. Trump is solvent because he doesn't bear any of the risks for his failures: he aggressively markets himself and his ideas to banks, which front most of the money. Donald Trump pays himself out of those loans. When the project inevitably falls apart or goes bankrupt, the banks are stuck with worthless property and Trump leaves with an exit bonus (or annual administrative and branding fees.) It is not immediately clear if this is a deliberate strategy, or if Trump simply does not have the money to fund his failures on his own.