Learn German: Die Karawane nebst den alten Wohnwagen ist hier. - The caravan along with the old trailers is here.

Phrases added by prepositions like nebst, samt, or mit do not change the grammatical number of the subject. The core subject is just the singular noun Die Karawane, so the verb remains ist. If you used und instead, the subject would become plural and the verb would be sind.

Why is the phrase den alten Wohnwagen in the dative case?

The preposition nebst always requires the dative case. Because we are talking about multiple trailers, the phrase is in the dative plural. The plural article changes to den in the dative, and the adjective takes the standard -en ending.

How can I tell that Wohnwagen is plural in this sentence?

The singular and plural forms of Wohnwagen are identical. We know it is plural here because of the dative article den. If it were singular, the masculine dative article would be used, making it dem alten Wohnwagen. Additionally, while dative plural nouns usually get an extra -n at the end, words that already end in -en do not get another one.

Could I use samt instead of nebst in this sentence?