Actually there are two ways. And I'm guessing your radio does not have a DVD player in it. But is the standard stereo.
One way. There are inputs on the front of most built in RV stereos. Run a split RC cord from the right, and Left TV RC out puts to the Right, and Left, RC inputs on the front of the stereo. Choose "AUX" on the stereo slider. The TV will play thru the stereo.
Two. Loosen the stereo, and gently pull it out of it's hole. There are the same RC inputs on the back. (this is the one I did). Route the cord from the TV thru the side of the stereo compartment, and plug them into the RC R, and L input jacks. Choose AUX on the front of the stereo. The TV will play thru the stereo.
Either way. You can control the volume with the TV remote. It just looks better if you hide the cord.