OK, According to the manuals for your electronics, you must connect the Audio Out left and right RCA plugs on the back of your TV to the Audio In left and right RCA plugs on the back of your Radio/DVD player. Then, by selecting Aud on the radio remote (button 25) you should be able to route the TV audio thru the radio speakers. You can then use the audio set up on the TV to turm off the TV speakers.
Your radio has 3 speaker pair selections. A,B and C. You do not have center speaker functionallity nor do you have surround sound. You can connect rear speakers and wire them to the existing harness on the radio. You can then use the built in fader to adjust speaker pair volume. However, it appears that you can only select one of the 3 options at a time. So, assuming your inside speakers are on A, you must wire rear speakers to the A circuit. B is probably your outside pair and C would then be unused. The radio would most likely have to be removed to wire additional speakers or connect the audio in on the back if you cannot get to it.
If you want real sound you might want to take a look at this:
Its a 12v system for that real "Boondocking" experience.