Learn French: En Corse, nous allons louer ce camping-car dont le propriétaire est sympa. - In Corsica, we are going to rent this camper van whose owner is nice.

For islands, French distinguishes between large islands (which act like regions or countries) and small islands. Corse is a large island and a French region ending in -e, making it feminine. Feminine regions and large islands take en without an article (en Corse). Small islands usually take à (like à Malte).

Why is there a le in dont le propriétaire if dont already means "whose"?

While dont often translates to "whose" in English, grammatically it acts like "of which" or "of whom". Because of this, the noun that belongs to it still needs its own definite article (le, la, or les). You are essentially saying "of which the owner is nice".

Is a camping-car just a regular car you take camping?

Not exactly! The word camping-car is a "pseudo-anglicism" — a word made of English parts but used differently in French. It specifically means a camper van or a motorhome (RV), not a standard car.

Is sympa a completely different word from sympathique?

Sympa is just the shortened, everyday version of sympathique (nice or pleasant). It is extremely common in spoken French and casual writing, though you should use the full word in formal situations. Even though it ends in -a, its spelling doesn't change to match masculine or feminine nouns.