I have a 35 gallon tank in the bed of my Dodge dually and simply love it. Best money I've ever spent on something that wasn't motorcycle related.
I love that I don't have to check out the fuel stations ahead of time to make sure I can get in, and, more importantly out, with that 40' toyhauler in tow. I can carry enough fuel to only have to fill up at the end of the day when the 5th wheel is not hooked up. Lowers the daily stress level considerably, especially if you're traveling the two lane highways with fewer big ol' truck stops.
I like taking breaks at rest stops, or parks rather than using the fuel stop as break time.
I like being on my schedule rather than the trucks fuel tank's schedule.
Okay, the cons...uh...well...hmm...it does cost more when I fill both tanks.