I don't think you'll be able to get hot water from your 12 volt system, so I don't think solar panels won't help you there. They will help to keep your lights, TV and water pump working for much longer than without a solar panel. If you're looking to extend your flexibility for water heating you should be looking at getting a refillable gas system - Gaslow or similar. You can refill at garages all over Europe without any difficulty, and there's no reloading of bottles as there is with Calor gas.