I added an RDS 45 gallon in bed tank to our '06 RAM a few years ago and then transferred it to our '15 RAM in the spring of 2015 shortly after buying it. I love the extra fuel I can carry! If I had one regret it is that I should've bought a slightly larger tank, maybe 60 gallon.
There are all sorts of configurations and brands of tanks to choose from, I chose RDS and gravity feed for it's simplicity. Yeah I have to manually flip the switch instead of switching it from the cab but I'm OK with that. I like being able to pick & choose where and when I buy fuel.
Dan