HAd a ford f350 that I traded in in Jan for a new Ram 3500, the Ford had a 40 Gal Aux Diesel tank, after having that spare fuel tank there was no way in hell I was not going to install one in my Ram.
As I looked for tank I wanted to stay under 50 Gal and also have a tank that sat below the bed rails as I have a Tonneau cover. Problem was 90% of the tanks out there sat above the bed rails.
I ended up buying a 46Gal Aluminum Tank from Tank and Barrel, the height was just below the bed rails and only used 12" of the bed. I went 46 Gal vs smaller as all 46 is not usable so I have a usable qty of about 41-42, and that applies to most tanks.
This tank made by Aluminum Tank Industries in Florida comes with everything you need (also included the Fuel Gauge) to DIY install. It only takes 2-3 hours to install and looks great in the bed. Now combined with the stock 32gal (which holds more like 36-gal) and the new usable 42gal I am pushing 80 gallons with a towable range of 750 miles before desperate for fuel. With that you can easily pick you stops.
This was the best truck addition I made and it is cheap (about $700 delivered) and easy.
Note: there are two types of fuel delivery, Gravity feed and electric pump, my ford was electric and the Ram gravity and frankly there is NO advantage to electric, in fact simply more to go wrong with electric, like forget the pump is on, pump failure, whereas gravity feed is easy and pretty much fail proof.
https://www.tankandbarrel.com/rectan...nk-p-1223.html
I will never drive more than 750 miles in a day and will mostly fuel after unhooked, however even if needing fuel between stops I can carefully select best options.
An Aux tank also end rang anxiety, we also have a Tesla and supercharges are generally 80 miles a part, we have a range of about 265 miles, this creates a fair amount of anxiety and planning for recharging, this is probably the ONLY downside to electric. However Tesla has done a great job of supercharger locations but there is still some anxiety when you get down below 30%. The Aux tank eliminates all Fuel anxiety makes traveling so much more relaxed especially if you are covering some serious miles