Learn Hardcore Croatian: Kamion stoji vani. - The truck is standing outside.

But in Croatian, just like in some English contexts, this verb is often used more broadly for something that is stationary or not moving. So with a vehicle, Kamion stoji vani usually means:

The truck is outside and not moving
often naturally translated as The truck is parked outside or The truck is standing outside

So the idea is not that the truck has legs and is physically standing like a person; it means it is there, stopped, stationary.

Why is it kamion, not kamiona or some other form?

Kamion is the nominative singular form, which is the basic dictionary form of the noun.

In this sentence, kamion is the subject, so nominative is the correct case:

kamion = truck / the truck

You would use other cases in other roles, for example:

Vidim kamion. = I see the truck.
Here kamion is in the accusative, but for this noun the accusative singular looks the same as the nominative because it is inanimate.

So in Kamion stoji vani, kamion is simply the subject.