I second what donegal says. I have an audio video custom installation business and I am a Sonos dealer, so take this with a grain of salt. Sonos has a lot of options that might work great for you, although it is all 110v based. Look at the Play:5 for out side. You can get two of them and set them up for left and right separately or get left and right out of one unit. You could use one or two of the connect: amps (depending on how many speakers you have in the camper) to power your indoor speakers.
www.sonos.com
One caviate is that the system is based on utilizing the Internet, so that will likely be a problem in most campsites (at least where I camp). What I would do is set it up at home and register it, etc. Then you should be able to take it on the road with you. Get yourself an apple express and set it up before you go so that you can 'AirPlay' music from your iOS device to one of the line inputs on the Sonos system. You will also likely need an Internet router seperate from the express to hand out ip addresses to the Sonos system and your phone that you are using for control, even though you may not have Internet.
It is quite doable but is a bit involved with extra parts and the like. How much time and money do you want to invest? There are more straight forward ways of getting good sound in there and outside, but the Sonos option is pretty trick, once set up.