Why carry more weight than you have to? I have a 40 gallon auxiliary tank and it works great. Adds about 300 lbs. I want enough fuel to not have to worry about stopping with the 5th wheel hitched up. I hate truck stops and not fond of driving around trying to find a regular fuel station that I can get in to, and, worse, get out of. Seventy gallons of diesel fuel will easily give me a day's travel with plenty of fuel left over, and maybe two days if we don't go far either day.
With a great big tank you're using fuel just to haul more fuel. That's JMO of course.