I can't think of anything that will flourish in the conditions you describe that wouldn't be taken care of by boiling. Toxins must be created by some organism, but if the organism itself can't start growing first, there's no risk of ingesting the toxin.

But you do mention algae. I don't have any experience with RV's or trailers, but can you actually get algae growth in the water tanks when maintained properly? I would think it would require sunlight to encourage algae growth.