Oh My! That didn't come out right...I think...
Newish Sprinters have 100L / 26ish Gallon tanks. 5.3 gallons are the "reserve" which gives about 20 gallons of "usable" fuel before the last bar on the fuel gauge disappears and starts flashing. 3/4 of 20 = 15. Leaves about 5 -8 gallons available to the Sprinter only. As I understand it, the generator pick up is set to 3/4 of tank. You might have as much as 18+ G or as little as 15ish usable to,the genny. At the campsite, it is what it is.... MINUS whatever you used getting there. Regardless, you may not have the mythical hours of generator run time with a diesel as the run time is limited by the actual fuel above the pick up. It is what it is....
I get nervous when that reserve warning fires up. I'm in Texas...it can be a very long ... very long... white knuckle drive watching that blinking light trying to get to a fuel point. So, I plan my fuel stop about every 300-340 miles, or roughly 20 gallons burned. That gives about 75-80 miles available on reserve... Just in case. Not keen on pushing this beast down the highway......