I’ve never heard specifics but a lot of evidence points to a form of folie a deux with his wife. I mean they both have mental issues but by joining forces they went turbo charged crazy. Evie always had a batshit reputation but she was a 1980’s model and you get the sense Randy thought he had really scored landing her as a wife - even if she was crazy and spent all his money.
I always got the sense Randy struggled with depression or some other issues but managed, and lived in the shadow of his good looking, coked out, famous older brother. Randy got to be the funny character actor. I mean shit, the guy started his career in The Last Picture Show, then The Last Detail and Midnight Express but ended up famous to most as Uncle Eddie from the Vacation series.