Sounds like you should have moved her truck for her first. It's just a car, and parking accidents happen, especially when a garage is involved.
Also sounds like you may have anger management issues if you got mad at the woman you want to marry over a little thing like that. It doesn't sound like it is something that will change the safety or operability of the vehicle, so why even care? This was your opportunity to show her that she's more important than a mere car, but you blew it. Buy her flowers and ice cream today to make up for it. If you're serious about wanting to be with this girl, you need to stop being defensive about your stuff and yourself from her. Defensive and angry reactions are almost never helpful in a relationship.
Girl cheats-->angry, and rightfully so
Girl accidently breaks something of yours that's expensive/irreplaceable--> be greatful for the opportunity to show her that she's what you really care about, not the object.
Ignore my advice as you wish, but if you do she may very well continue to be hurt by this incident, and afraid of you, for years to come.