I replaced the cheap outside speakers on our trailer with these 
PolyPlanar MA-905 marine speakers.  The challenge is getting a decent speaker that will fit in the thin wall of an RV.  
I mounted these so that the woofer sat in the hole from the factory speaker and they fit perfectly.  Decent frequency response (55hz on the low end) and plenty of capacity.  I added a Kenwood 200W power amp with speaker-level inputs after the factory head unit and you could hear these halfway down the beach! 
You can get them for a good price on 
Amazon.