I run a RIDUCULOUS amount of sound on the exterior of my bus. I've been in the pro audio, marine , commercial sound industry for 4 decades now…. If ya want to do external speakers to run of an existing system (a hub with all your sources, power +maybe on a solar and not have to be within 30 feet to run a BT signal) then yes external speakers are a nice solution but $$$$ to do it right. For $75 you can get a decent all weather splash proof box & run off your BT music player you keep near…. and use anywhere you are willing to also bring the iPod/phone.
I would recommend a 2-way speaker with at least 8" woofer for good mid bass ( your not gonna get "deep bass" thumping on any compact ext speaker) with a compression horn combo for distance clarity….. you can use an internal volume control for levels or an aux amp on a switch with adj gains. look for a 2-4 Ohm configuration for efficiency if using like a "car-audio" set up or run 2 8ohm speakers mono off a bridged amp…. make the speakers removable or someone will remove them for you! (also you don't wanna look like an ice cream truck)… by the way you will most likely piss off most of the forrest with "your tunes/volume"…….lol best bet is the simple one…… a small portable, I do own a couple ION "tailgaters" plenty loud too…. BT speakers are getting cheaper everyday as well…… good luck on your musical quest… just keep your usage application & budget in mind!
(I also mounted speakers below my vehicle as they are not as visible and closer to the listening area = less power/volume required)
your application may vary!