Historically, although a few do, most trailers do not have a cable from the TV audio output back to the surround sound. Yours may or may not have a cable.
Assuming a cable is there, you may have to adjust the settings on the TV menu in order to send the audio, and you may have to figure out the correct settings on the sound system to play that audio. This varies a lot by TV and by surround sound - there's no single answer.
Your starting point is probably to verify the cable, or if necessary to install a cable. You may have to pull the surround sound to see what's plugged in, using the manual as a guide to understand the various connections. You might have to also check what's plugged into the TV.
If the cable was not supplied, as on most trailers, to get the right cable, you'll have to determine what type of audio outputs the TV has, and what type of audio inputs the surround sound has. The manuals are the place to start to determine the connection types. Often there is no matching connection at each end, in which case adapters may be needed.
We have an owner-written
RV Audio Connections Illustrated guide that will help you figure out how to bridge connector types.