My reply was based on the wiring diagrams above and the parts book for a 2006 Adventure G38J indicating you have:
TV: Sony model 27FS120 (http://www.docs.sony.com/release/KV27FS120rev2.pdf
Rather than run around in circles I used snapshots from those manuals and and the winnebago wiring diagram to make:
Note when reading the following closely read the wording of Composite video (Video 1 at TV) vs Component Video (Video 4 at TV)
As you can see:
1) The audio output from the TV is being feed to the Video 1 input on the DVD which is being usd as a audio only input channel. Both Charlies and Dougs rigs are wired this way. Doug however does not have a Sony DVD and I could not locate a pdf version of the Memorex unit. Per the directions in the coach operators manaul, you have to use the set up menus on the TV to select Varible Audio output. Procedure is in the TV manual. For Charlie, on the DVD player remote, you have to select the Video button (not DVD) and then use the input button to select Line 1 (Video 1). That will route TV audio out through the Ground Loop Filter then into the Sony LIne 1 input. You can then use the DVD volumn control (or TV volumn since Variable was selected) to adjust the surrond sound volumn.
2) Per the Sony manual, the Sony DVD unit will only supply DVD video out over the Component video path (the green one). Unfortunatly, the TV was designed such that the Component Video input path (Video 4) has seperate audio inputs. The Sony DVD only provides 1 set of Video channel Audio outputs which the Winnebago designer only routed to to the Video 1 input channel (with composite video). With no audio input to the TV Video 4 channel you CANNOT hear the DVD audio at the TV. You could get two "Y" adaptor cables and feed the audio from the DVD to both video 1 and 4.
As I hope you can see, you have to perform set up procedures at multiple place (TV, DVD, Control Center) in order to route Video and audio information through the system. I wish it could be simplier but when using seperate componets it becomes a necessary evil that requires you to understand how each device is connected and setup for various purposes.
Good luck (I rest my case