I have an ipod Touch, keep going back and forth between a car stereo system (head unit) or a portable radio/ipod unit.
Altec Lansing has a fairly decent sounding unit with a small footprint, and affordable price tag. Amazon.com: Altec Lansing iMT620 inMotion Classic Portable iPod Dock with Rechargeable Battery and FM Tuner: MP3 Players & Accessories
If you want enhanced audio sound, you'll probably want to go with a car stereo system that accepts your iPod.
They are now coming out with "head units" which include a built in USB receptor. Using a USB cable, you can plug your iPod directly into the system and it will charge your iPod and play music at the same time. The audio speakers are fairly good (as with the "head unit" the more you pay, the less distortion), and they are designed for travel.
It is a 12 volt system (for us boon dockers), and you get FM/AM radio (which is important to me when on the road).
Remember you will be cutting into wood, so the big question: do you want the speakers to face down (onto your dinette or bed) or face outward towards the rest of your cabin ... or both?
Also, where do you place the equipment with respect to your power source? And ... audio needs.
This isn't really a portable system, so you will need to think about how long you will have the trailer. If you are starting from "scratch", and decide to have a audio shop do the installation (which involves cutting into wood) ... the install will cost as much if not more $$ than the head unit and speakers.
BTW, I like the marine speakers. It's just that they seem bulky and I'd rather have built in speakers.
I would install a auto audio or marine speakers over computer speakers simply because the former are designed for road travel.
Another great thread!