A 3.5mm to RCA cable is very handy, but I don't think it does what you think it does. At least not in this case. Tell me what the colors on the RCA end are. Is it just red and white? That's only for audio. Which if the TV/ DVD deck in your truck is the only AUX input you have-- that's the way to go to play audio from your phone (music player/ streaming radio) on your stereo system. It's how I have my parents' Excursion set up for them.
A 3.5mm to RCA cable is certainly capable of delivering video. Many consumer video cameras used this port scheme. If the cable supported that, you'd see three rings on the 3.5mm side, and also a yellow plug on the RCA side. But to the best of my knowledge, the S4 (nor any other Android device) doesn't deliver video from the mini jack. Just from the micro USB port, which requires an MHL adapter.
Does the deck in your truck have an HDMI input? If it's not fairly new, it probably doesn't. If that's the case, then getting video to it isn't going to be easy. You can't directly convert an HDMI plug (digital) to RCA (analog). It requires a powered conversion box to covert the signal that costs probably a couple hundred bucks.
Back to the audio-only talk, if you're liking streaming music or playing music from your phone in your stereo, I know an extra device that you might like. It's called TuneLink. Basically, it plugs into your power point in your vehicle.. it looks like a typical phone charger with a USB plug (and can be used as one-- just not for quick charge). Your phone connects to it through bluetooth. Then it has a 3.5mm output on it. So the chain goes
Phone audio ---(through bluetooth)---> Tunelink ---(through 3.5mm to RCA plug)---> AUX input on your truck
TuneLink Auto hardware accessory for iPhone, iPod Touch & iPad
I recommend the "iPhone" model with the blue light, over the "Android" green light model. There's not really a difference.