YOu need a food grade container to store RO water for a long time. You could buy large polyethylene bags (food grade) and put 2 sheet but I would be very worried that eventualy there will be some leak. It's your call.
Polyethylene and polypropylene are both food grade and I bought those large polyethylene bags before to hold sensitive limited edition prints because they don't leach anything. I would not use regular garbage bag at all. Polyethylene are easy enough to order via the internet. Search Google for polyethylene bag food grade.
You should use fine sand paper inside the jar so that there is no small bumps or little imperfections inside that could pierce the bags once they are filled with water and put a lot of pressure.
Here is a company that probably sell food grade bags that would be safe and would not leach chimical into your aquarium:
http://www.interplas.com/poly-bags