The biggest issue you will have in mounting speakers outside is getting the speaker wires routed from the stereo to the location where you want the speakers mounted. Once that part is done, mounting the speakers should be an easier task. You'll find your speakers types will either be surface mounted or recess mounted. Whatever you get, make sure to get 'marine' speakers. they are made from materials designed to withstand exposure to the elements.
I am planning on getting rid or the crappy outside speakers on my 2013 Eagle 284BHS. I just ordered some Polyplanar MA905W
They are relatively flush mounted and require only a minimum of depth for mounting. From what I can tell the inside thickness of the wall is about 1". The mounting depth of my new speakers is 1". So they should mount within the thickness of the wall without having to go completely through to the inside. If needed, I can fabricate some nice looking shims to raise the speaker a bit away from the wall, but the shim won't have to be more than a 1/4".
FAA licensed A&P aircraft mechanic, ex-RV dealer parts dept
Retired B-52 Gunner/Flight Examiner, B-52 Crew Chief
2016 Jayco Eagle HT 29.5 FBDS w/ 15,000 BTU AC, 50 amp service, upgraded counter top, thermal pane windows, elec. stab jacks, outside grill, backup camera and MORryde pin box.
2011 GMC 2500HD DURAMAX/ ALLISON