X2. You will hate life if you run out of fuel in a diesel rig.
I'd do as above -- 1/2 tank, start looking for an easy to pull into place to fill up, 1/3 is to find it ASAP, and 1/4 is when you pull over, grab the Jerry can from the hitch mounted cargo rack and empty that in the tank for 3-5 more gallons of range. (I have a habit of carrying 1-2 5 gallon fuel cans with me and rotating the fuel out at least monthly.)
Sprinter chassis tanks are tiny. You have a 26.4 gallon tank, which means you have about 23-24 gallons of usable fuel. On a long trip, if you are running toadless, I highly recommend a hitch mounted carrier and at least one 5 gallon diesel container, if not several.