Learn Swedish: Tänk att husvagnen förstörde hela bilresan! - Imagine that the caravan destroyed the whole car journey!

While it literally translates to "Think that," it functions as an exclamation meaning "Imagine that...!" or "To think that...!" It is a common expression used to show surprise, dismay, or disbelief about a situation.

Why is it hela bilresan and not den hela bilresan?

The adjective hel (whole/entire) breaks the usual double-definite rule. To say "the whole" of something, you use hela followed directly by the definite noun. You drop the freestanding article (den or det), which is why we say hela bilresan.

Husvagn is made of ett hus (a house) and en vagn (a wagon). Why is the definite form husvagnen and not husvagnet?

In Swedish compound words, the gender is always determined by the final noun in the compound. Because vagn is an en-word, husvagn is also an en-word.

This lesson covers emphatic om-clause responses. How could I reply to this sentence using one to strongly agree?

You could reply with "Om den gjorde!" (Did it ever! / It sure did!). By isolating the om-clause and echoing the action with the helping verb gjorde (standing in for förstörde), you emphatically validate the speaker's frustration.