You can always replace it with some square aluminum or steel tubing of an appropriate size. I was going to suggest angle stock but that would be tough on your legs:
https://www.homedepot.com/b/Hardware...0square%20tube
Another alternative would be to make new rails out of hardwood like oak or plywood of an appropriate thickness. If you make them 2" - 3" high, I doubt that would ever split.
I suspect you just had a bad piece of wood. In all my years of RVing, two TT and now a MH (and many more years of sailboating with similar beds), I've never experienced this and I'm not a lightweight.