First of all, I don't believe you can reliably clean a milk jug of all milk residue. Even a little bit can open you up to serious bacterial contamination. Even if you can , why take the risk. Bottled water is cheap and you can safely reuse the bottles. The last thing you need in a water-short situation is a is case of screaming diarea !!!
You are risking alot of bacterial build up from the milk any long term storage with milk jugs is bad news. Get a Berkey filter if you can afford it if not I have read alot of people talking about pool hypoclorite on survival websites for emergency water. If your wanting to go for the real short term as far as days prepared to go without water for I would definitely drop the dollar a jug it takes to get sterile jugs at the store. At a minimum one gallon of water for DRINKING a day and thats not counting your washing etc.
I too have had bad results with milk jugs. I have had excellent results since switching over to the heavier plastic juice/tea jugs.
For Wash water, water for things other than drinking, I save the laundry detergent bottles.
Viking Queen
We have 12 that we rotate very often. Over the course of about 10 years, we've had 3 or 4 "leakers". Sometimes those were due to other factors...