PEX can expand to near 4 times its size. The only way I see it cracking is if it froze, and even then most of the time it wont. It shouldn't ever in your case as your radiator fluid should never freeze. If it did you'd be in a world of hurt. It could leak like anything else, but most likely will not. Most of the folks on Skoolie are using PEX for their water systems. More versatile then copper. It's flexible and awesome stuff. Plus you'll only have two open ends on the hose and both will be under your dash. Easy repair if it leaks there. Your bus though. Just think about it because it's a great lititle addition if you're going to be living in a cold climate.
Roads? Where we're going, we don't need ... roads.