I just used some old/ unused (decent quality) equipment I had to build my own sound system. I'd bet you can find an old receiver/amp and some speakers at a thrift shop that'd put that factory stuff to shame.
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former 2008 Jayco Eagle 29.5 RLS
former 2014 F250 6.7 4X4, CC LB
Anyone that has had the outside speakers and a set of the inside speakers removed, is the outside a higher ohm speaker than the inside? That could be part of the difference as well. While the stereo could very well be a lower output rating for the outside, changing to an 8ohm speaker for the outside will also reduce the power compared to a 4ohm speaker.
I still have a set of MB Quart marine speakers in the box. If they are any good still I wonder if they'll fit.....