Have you considered HDPE plastic for the water tanks?
As for fibreglass diesel tanks, I know they exist, but you need to research whether the current trend in mixing bio-diesel or other supplements to straight diesel can affect fibreglass. My limited understanding of this is that epoxy
fibreglass is better for fuel
The advantage, of course, of "fg" tanks is that you can simply build then out of encapsulated plywood and then glass in a cloth interior in order to make them "form fitting".