On my XLR, the outside speakers (kicker 6") are under the kitchen cabinet with the speaker hole through the thin wall, with a abs speaker box sealing it. A decent speaker will not fit within a 1-3/8" wall cavity. It might be possible to surface mount them with some kind of thick spacer.
Like some have said, the head unit could be far, far better. But, I would first make sure the volumes / balance / fader is set correctly and if they are; try wiring in a small (around 25W) amp to the factory outside speakers and see if that is all you need. You could tuck one of these almost anywhere.
If your speakers aren't as good as these low dollar Kickers, I have, buy a set and install them (after you try the amp). High quality "audiophile" speakers on the side of the trailer is a waste of money in my opinion. I spent the money inside.
The original "system" was a $40 head unit and two sets of 6" Kickers and is the cheapest and poorest so called system I have ever had in an RV. I bought it knowing the original junk was going in the trash.
I am considerate of my neighbors, but believe me, during the daytime I can hear it from as far as I need to.
XLR Hyperlite 29HFS
Ram 2500 6.7L CTD
A Modder's work is never done!