How do you keep your dog food?!

Ok, so I don’t want to harm my dogs, but I am running out of ideas. I buy 120 pounds of dog food for the month. This is so I don’t have to go to the store once a week. Now every website I have been on has said to store dog food in its original bag because it helps preserve the nutrients. If not that, plastic or stainless steel. I would use stainless steel but my main problem is that I like to mix their dog food. This is because I would hate to eat the same thing every day so I mix their food. I saw something that said “do not mix dog foods”. One said don’t mix like don’t poor your new bag into a container with your old bag. So would filling a container with all three bags at the same time be fine? How do you guys store your dog food?

(I have an appointment with the vet to ask but its not for a bit) ps I have six dogs and that’s why I buy bulk and don’t go every week