I have no idea how I lived without mine.
105 gallon (white steel, I'm cheap and it's a white truck) L shaped tank, with the regular tool box on top.
Still fits in the gooseneck no problem (8' bed of course)
Has things (?) so the fuel doesn't slosh around and it's great
Got an electric pump because the wife is a wimp and to be honest, i like it a lot, you're tired, you've been working hard, it's hot, just flow some diesel.
BTW, diesel lasts pretty much forever, doesn't go bad like gasoline, having extra is no big deal.
I fill the truck occasionally with it (I don't buy dyed fuel) and when I get down a bit and I find a deal I fill everything up.
For like $300 plus dollars
But it's great
Get a gallon meter on it, so you know how much you have run thru it (I just guess) and put a fuel filter on it, that would be my only two changes.