In my selection... I too wanted a unit that played mp3s and had bluetooth. Being waterproof was my prime consideration which narrowed it down to the Zumo. Then was a matter of whether I wanted satellite radio/weather or not. I ended up going with the Zumo 660.
To tie it into the audio system on the RSV, you'll want the Buddy Rich cable. Not only can you play mp3's and hear them and directions thru the headsets, bluetooth sync with your smartphone allows you to make and receive phone calls. Bluetooth will sync your smartphone address book with the Zumo so you can search the gps address book for names (which get wiped off when you disconnect between the units).
Pricey but being waterproof, with the features it has, and an easily visible screen in sunlight... worth it.