Only because I like you Jack, I went out and took some pics! My mother bought me a DeeZee 40 gallon gravity fed aux tank for my birthday from Amazon. I did the install myself. I don't think I'd want any bigger of a tank. It's about my limit for when I want to stop. I've been very happy with it and frankly should have done it sooner. I also purchased (at Tom's direction...at least that's what I told my wife) an electric solenoid that I wired to one of my up fitter switches. When I get to about half tank on my fuel gauge, I push my up fitter switch and watch my gauge slowly go back to full...It's awesome!! Here are some pics, so you can see some of the space that I have left between the tank & hitch. You can also see where the fuel cap sticks up a tad into the tonneau cover.
EDIT: I see Tom beat me to it. Also, there's a ton of install video's on Youtube for this sort of thing.
'16 Ram 3500 MC CTD Dually, 4:10 rear
'14 Ram 2500 MC CTD (gone)
'14 287BHBE (gone)
'16 North Point 377RLBH (current and hopefully last)