I use an FM transmitter for two purposes in my trailer.
1) I don't have outdoor speakers and I like being able to listen to the same music inside my trailer and outside my trailer. I simply plug my iphone into the FM transmitter and then I turn the trailer radio to the desired frequency. I then have a portable radio outside that I also tune to the same frequency. That way I get the same music playing both inside and outside. It works really well and I've tested the signal to about 150' through my real home exterior wall. Not a drop of static.
2) If I want the TV in my trailer louder I do exactly what you want. I take the transmitter out of my iphone and plug it into the headphone out jack on my TV. The FM transmitter then receives the audio from the TV and broadcasts it out. I just have to set my trailer radio to the same station and I'm golden. It works really, really well.
Of course, you could run cables from the TV to the radio but that requires more work. Here is the transmitter I have. http://www.ccrane.com/radios/fm-tran...x#.UFKAf1Egj8k
One word of warning though. Out of the box, the transmitter doesn't go very far at all. All you have to do is unscrew the backside of the transmitter, turn a single screw a bit and the broadcast range increases dramatically. It's not hard to do at all, ANYONE can do it. I can give you directions if you want to go that route.