On a truck I can't say, but on a car, if well designed, you should be able to see left mirror (right on UK or similar right lane driving) just turning your eyes, not your entire head.
Although people that use glasses (for example) tend to always turn head instead of eyes because is how they will see better. It's a reflex non conscious act of that people compared with the ones that had never used glasses. Even people that had stopped use glasses (uses contact lenses now or has fixed their vision with surgery) keeps that unconscious head movement for a long time.
Whatever, what it's clear is that a mirror will NEVER be placed in our front, they are at a side, near but at a side anyways. So, it's naive to expect to see the left mirror when looking straight into the game on a regular single-display desktop setup. As Max said, that should require the whole 3D scene would be distorted a lot, like if we were on one of that mirror houses on a theme park.
If people don't want to turn a bit the head with whichever option the game provides, there's always the F2 mirrors.