If your RV radio has aux input, a 3.5mm to stereo RCA adapter link linked above is what you need. If you do not have a 3.5mm aux input on the front of your radio, you may in the back. You can pull that out and look for what type of connectors are on the back. You can find a clue in there is any kind of aux input "mode" (like FM/AM/CD/NOAA/AUX).
If there is no aux input, you may just need better speakers. Computer speakers systems are powered and reasonable cost. My RV sound system sucks. I thought about replacing the no name radio/DVD player, but then the speakers still suck. So I picked up a portable Bluetooth speakers which has been used almost daily between home and camping.
A & A parents of DD 2005, DS1 2007, DS2 2009
2011 Suburban 2500 6.0L 3.73 pulling 2011 Heartland North Trail 28BRS
2017 Subaru Outback 3.6R
2x 2023 Chevrolet Bolt EUV (Gray and Black Twins)