Jumping back to the original post, you said you plugged a cable in from a set of connections next to the DVD player to the TV, and then from the front of the DVD player to the TV?
Those are generally Inputs, not Outputs.
If we're talking about one of the all in one RV units (like the Genesis or Jensen, etc..) then I'm willing to bet the sound you were getting was coming from the trailer's speakers, which are hooked up direct to the player, than from the TV.
If that's the case, you need to see what they installed to hook up the output of the player (should be on the back of the unit) to your TV.
There's no use crying over spilled milk, unless it's on your keyboard.