The best place to ask is over at the Expedition Portal site (Expedition Portal
) as they are experts on Overlanding. I have been building an off road trailer for the last year, so I have researched this exact topic. What I would do (am doing) is the Eccotemp propane water heater (Eccotemp L5 Portable Tankless Water Heater and Outdoor Shower - Portable Hot Water Heater - Amazon.com
) and using a water pump such as a Shurflo 12V. There are other models, but the principle is the same. Forget solar showers, forget rooftop "tubes" and all that crap. This system is good in any weather and if you need more water, you don't need to wait to have it get warm. The only power you will need is for the pump and you can supplement the battery with your solar panel.
If you have running water from a hose, that can go straight into the Eccotemp and you don't need a pump. For that you just need the propane. Otherwise you need pump/battery, a resevoir, and the Eccotemp. Plus it is cheap, under $120.
Here is a sample set up.