We've been using the collapsible 5g thing for a year now, and I am starting to think about using something different. The nice thing as others have said is that you can flatten it and store it in the external storage behind the fridge. You can actually slide it up in the compartment a little bit to leave more space there for other items (just don't slide it up all the way to where it will inhibit heat exhausting from the fridge). It's also cheap.
Here are the cons that I have found with it:
Having to burp it to that the sink will drain. I kind of understand having the one way valve on the hose, so odors won't come back up through the sink, but to be honest, our gray water smells mostly like toothpaste and mouthwash, with a little pasta water and wine mixed in.
Poor structural stability. Any time it is close to full, it becomes a three stooges skit trying to deal with it. On our last trip, I tried twice emptying the grey water into pit toilets since we were in some bear areas. Usually, I drain it into the fire pit, but this is not "bear aware", so I wanted to see how it would work putting it into the pit toilet. Both times it was a disaster. I could not find a way to do this without hosing the bathroom down with grey water. It just geysers out of there because of it's own weight and the flex of the material. The first time we used it, I thought I would try the "lid with a handle" that came with it. As I was installing it, I thought that there was no way that the threads would support almost 50 pounds of water. I got about 10 feet before the lid popped off, the bucket fell to the ground, and old faithful showered me.
Transparency. This is a silly gripe, but when the grey water is of a questionable color, I wonder if tent campers walk by and think that it is actually a black water tank.
Anyway, I see three alternatives: Bed rail PVC tank, rigid water jug, and 5g bucket with a lid. I think the bed rail PVC tank is the most convenient, once built. I don't want to drill holes in my bed rails to mount it. Why don't people install it on the camper itself, instead of the truck?