Companies don't want you to know this, but “right to work”/”employment at will” is not absolute. You are still protected from retaliatory firing, you are still protected from discriminatory firing, and they still need to cover their butts with a paper trail documenting reasonable cause to fire you.
They like to say things like “we can fire you any time we like” and “you're replaceable” to scare you, but they actually can't just fire people willy nilly. They have to have a reason. The main difference is that you don't have a union to protect you.
Check your employee handbook for references to a “probationary period”. If they say that, even if they follow it up with “this does not alter the at will nature of your employment” it does. Courts have found that if your employer has a probationary period, that means that once that period is over you are no longer an at will employee. HR law is fascinating. The people running the classes are almost always gigantic AH who talk about the poor little corporations being screwed over by greedy disgruntled former employees, but I can't recommend learning a bit of HR law highly enough.