I've got some bad news, GET BOTH
Solar is great and if the size of your panel is fine you will have no trouble,
But you get the days when solar conditions are not good and you will need to charge the batteries, don't rely on the car to do this because even they fail.
I have got a 250watt panel and a 1kva Honda generator and a Ctek 25 amp charger, the Honda will run the Ctek and give full 25 amp charge into batteries when needed, and for those days when even the car battery is flat put in on and it will have charged it enough in about 3 mins to start your car.
The 240volts also comes in handy for all sorts of things , drills lights etc......
As for the shower cubes don't do it I have tried both of them and find then quite troublesome
Go onto eBay and get an instantaneous hot water unit ( paid around $199.00 for mine) and it come with shower attachments .
I've mounted mine inside my caravan, my brother in law has his mounted in a box on the outside of his van, and we even use one loose to take away on simple camping trips.
Hope this helps BOB