The tank on the back is a four gallon one from tourtank.com. I had it and my stock Suzi-Q tank last summer on a 4,200 mile run to Miami, FL and back. It really makes a difference when you stop when you want to and not dictated by having to find a fuel stop. I had it powder coated, they come in a burnished aluminum and in a few different sizes that looks like a mini beer keg. The mounting bracket is included as well as several choices of fixtures and hardware for however you might want to mount it on your bike. I just drilled two holes through my sissy bar and used two of the four mounting holes on the tank's bracket. then put the seat backrest back on over the bolt heads. You can pick the back of the bike up with it if you have to.
My one minor complaint is that even with a vented cap it still will splash gas out around its rim if you fill it anywhere near full. I just checked their web page and they've added a vent/overflow tube that might solve some of that problem. With it and the five gallon Harley tank on the front I'm good to go just about anywhere with no worries about being stranded between gas stops.
www.tourtank.com/Jack