The bathroom vent had a fan in it at one time. Could have been removed by previous owner if it's an older trailer. If this is a new trailer then you need to have the dealer replace the fan and make everything right. The fan is necessary to exhaust moisture from the bathroom as well as keeping everything "fresh as a daisy".
If you need to do it yourself, make sure you have 12v at the wires then buy one of these:
https://www.amazon.com/Hengs-90043-C.../dp/B002N1J5IM
I think these are a great drop in replacement.