[on or in] the bus

I have made up the sentences below.

(1a) There are twenty seats on the bus.

(2a) There is a surveillance camera installed on the bus.

My non-native English speaking friends think the preposition is wrong. They suggest "in".

(1b) There are twenty seats in the bus.

(2b) There is a surveillance camera installed in the bus.

I think my friends are wrong because the seats and camera are inside the bus. They disagree with me because "on the bus" is a common collocation.

What is your opinion? Thank you very much.