As others have mentioned:
- if your around cities you can use the radio (stations go in/out as OP mentioned)
- if your on most 'major' highways you can stream with your cell phone (until of course your out of cell phone range and of course it uses DATA which can be costly)
- if you can find a place to mount it, you can use XM radio but have to listen to whatever they are broadcasting (and its not cheap)
- while cars/trucks have moved from having just AM/FM to radio+cassette to radio+CD player to radio+CD player+Bluetooth you could simply feed CD's (with just audio or +10 CD's worth of MP3's stuffed on it) or you could load your cell phone up with a bunch of MP3's (most cell phones these days have at least 64GB of storage - 100x what a CD holds) and stream for free from your cell phone.
There are a BUNCH of options for car/truck audio - why your asking in a Truck camper specific forum is anyones guess...
- Mark0.