I'd think about when, where and why you want to listen to music?
When-- daytime, or after hours? Where-- in campgrounds or in the boondocks? And, in or outside your trailer? Why-- as background filler? A hi-fi experience? Head-bangin', guitar-shreddin' glory days?
A good car stereo and speakers can be bought for a song-- pawn shops are full of even cheaper ones. But they won't power the headphones you might want for late night listening beside a sleeping spouse (if any). Internally mounted speakers aren't much good sitting around the campfire, either.
I'd look at a range of modern portable radio-iPod combination units that could be moved around the trailer and campsite as the mood strikes. Like this one Tivoli Audio - SongBook® AM/FM Alarm Clock Travel Radio
Tivoli radios have excellent radio reception, sucking in distant signals in remote spots. They offer very good sound quality (though no tone controls) and good connectivity with other inputs and speakers. The portable models run on rechargable AAs, so no power connection is needed.I think this, plus an iPod and quality headphones, would be my choice.
If you still want permanent speakers inside, I'd shop desktop computer speakers, which should be powerful enough inside an egg. Quality correlates with price here, so aim accordingly. There's probably never been so many choices in smaller speakers, but most are lower quality. Those WM speakers pictured look pretty lo-fi, intended for "Now hear this" duty.
Boxed speakers will probably sound better than a raw speaker mounted in an un-engineered, overly resonant fiberglass cabinet. They also give you more flexibility in placement. Happy shopping-- and remember, listen first!