Learn German: Der Zirkus samt den gewaltigen Wohnwagen ist eine echte Sensation. - The circus, including the massive trailers, is a real sensation.

The preposition samt, which means including or together with, strictly requires the dative case. Because the trailers are plural here, we use the dative plural article den and the dative plural adjective ending -en.

Since the sentence mentions a circus and multiple trailers, why is the verb ist in the singular?

The grammatical subject of the sentence is just Der Zirkus, which is singular. The phrase samt den gewaltigen Wohnwagen is a prepositional phrase acting as a descriptive add-on, not a second subject. If the sentence had used und instead of samt, the verb would have to be plural.

The word Wohnwagen looks like a singular noun. How can we tell it means trailers in the plural here?

The noun Wohnwagen does not change its spelling in the plural. You can tell it is plural here by looking at the article den. If it were a single trailer, the dative case after samt would require the masculine singular article dem.

Could I just use mit instead of samt here?