Here I am, absolutely SHOCKED that we haven't seen the photo of the yellow 5 gallon can in the truck bed........
I installed a 60 gallon RDS right after I bought my truck in 2011. And while the main purpose of buying it wasn't saving money on fuel, I am absolutely certain that it has more than paid for itself in the time we have had it.
We went over 1000 miles two weekends ago towing the boat, and having the tank saved me two fuel ups where the prices were more than $1/gallon more than the fuel I bought at home.
Sure is nice to stop where you want to, not where you have to. I'll always say that the aux tank was one of the best upgrades I've done to the truck.
And, FWIW, around here the Krogers allow you to purchase 35 gallons at the earned grocery discount. And at least at current prices, I can get the full 35 gallons prior to the pump shutting off due to a spending limit. I hope that continues through the summer, although I have my doubts.