If you are asking me - I have a 2012 Ram 3/4 ton pickup with the Cummins 6.7L diesel and a 35 gallon tank,
Depending on several things and with us always staying on paved roads. we get between 10 -13 MPG. So we are good for about 350 - 400 miles. But we always plan our fuel stops at a Flying J so when we get between 1/4 to 1/2 empty, we stop for fuel.
EPA requires gas tanks to have carbon vapor canisters, diesel has no such requirements. There are several companies selling aftermarket under bed oversize diesel tanks, I can't remember a single gas one for something newer then mid 70's.
My 62 gallon under bed Titan tank is good for about 1000-1100 miles empty, 500-550 with the TH. It does go from 1/4 to empty in about 50 miles tho...
Actually, according to this "Except for diesel fuel tanks, the fittings through which fuel is withdrawn from a fuel tank must be located above the normal level of fuel in the tank when the tank is full. "
Which means that gasoline cannot be used in gravity feed tanks. It can be used in tanks that are pumped from the top.
2005 Newmar Dutch Star 4024 DP 370 Cummins
Towing my 2012 Malibu w/Blue Ox towbar & Patriot brake system
Here is another BIG reason, in certain parts of the Western US, it is NOT uncommon to drive 100+ miles between ANY place that may have fuel, depending on your flavor,there is not a fuel/gas station every 20 miles or so like the Eastern half of the US and traveling up North is much the same way.
The 50+ gallon bed tank makes a great safety item in that aspect, + the 35 gallon OEM tank and I can choose where and when to fuel up etc.
2010 F-350,6.4PSD, 6spd man trans,CC,SWB,SRW, Caravan camper shell,50 gal bed tank,17,000lb Husky WD hitch,Northwoods 2008 28KS Desert Fox Toy Hauler,2005 Honda 500 Rubicon ATV w/rumble seat,1 Aussie waiting,watching and ridin shotgun on the whole outfit.