I installed a Kenwood DDX6904S that I pulled out of my F350 when I traded it in to replace the craptastic Furrion "entertainment center" that came in our Outdoors RV 24RLS.
Huge improvement is a gross understatement. It has DVD, Bluetooth, Satellite/ HD radio, USB, Navigation etc. Not to mention 50 watts of power vs the Furrions 12ish. Sound quality is amazing.
I then upgraded the interior speakers to JBL 522's and the exterior to Fusion Marine speakers (cant recall the model offhand).
Again, huge improvement is a gross understatement.
It is well worth the time and investment if you enjoy music. The OE entertainment and speaker systems are capable of producing noise that resembles music, that's all they are capable of.
As mentioned Crutchfield is a great resource for your sound needs and the staff is quite helpful guiding you to affordable solutions for RV's.