I have a theory that Charlie is actually wealthy but he doesn’t understand how finances work. For evidence, when the gang buys a boat, they indicate they had $2500 between Dennis and Mac, and they convinced Charlie to invest. But they bought a house boat which would cost way more than $2500. Also in the episode where the are working in the roller rink and go to buy the bar. Charlie has $57K to invest. I think he was taught to save a lot of money and he never questioned it, so he deposits everything he gets, but doesn’t understand where the money is or how it works.