That is why I was saying I have seen some mounted in the door of the storage area. Ill look around google images and see if I can find a photo where someone has done this...
these trailer walls however are not very thick and you can come up with several ideas to either mount speakers or at least add outside speaker jacks in a flip box like the cable TV is using to gain entrance. Then you could just plug in a set of speakers and hang them on a screw head close by...
My speakers on my OFF-ROAD POPUP are water tight. I see see the back side inside a cabinet on the inside of the trailer. Have never seen any water come thru it...
Mounting cables is a trick sometimes but usually you can get to where you want to go and still be inside a cabinet somewhere.
If you have to go outside the cabinet along the walls there is WIREMOLD cable management strips from LOWES that can be painted to fit your decor and are excellent for using for RG cables, extension cords, video cables etc from point A to point B... looks something like this...
LOWES has a package of 12-feet of WiREMOLD with a bunch of corners and like items CORDMATE II C210
I always enclose my interior wiring in something like half-split plastic conduit or whatever. I also love to use PVC electrical boxes when i have to through a wall or cabinet side etc... Always looks professional instead of just poking a hole through the walls... many places to hide your wires and still looks good - My wife is an excellent inspector for my projects...
Type in "RV OUTSIDE SPEAKERS" into google search and when it loads up select IMAGES from the left side to find many many pages of RV speaker photos and links... Might find something there to catch your eye. there was several images of an outside speaker box you could mount to the side of the trailer and just run a small cable through the wall and not have to cut a large hole in the trailer side.