i'll be interested in seeing others responses here as well - in the interim i'll share what i had in mind
first, there's a good thread with shots showing the existing fuel tank as well as the available spaces under the floor and some preliminary measurements of the available cavities at
https://sprinter-source.com/forums/showthread.php?t=1348&highlight=spaces
my thoughts, as i want to do the same, for not only extended travel distance but for fuel for generator, was to install an additional 8 - 12 gallon tank under the floor, and simply plumb it to "T" into the fuel return line of the existing tank, with a small Facet 12V fuel pump switched at the dash, so i could, as needed, pump additional fuel over into the stock fuel tank. Main reason for me was the fact that, assuming there wasn't an aftermarket one already existing & offered, that would be easier than re-creating the fuel sender/pump fittings that already exist and second, that would be the least intrusive on the existing fuel system, ie for warranty considerations with Dodge/MB.
I was planning on putting it in the cavity behind the differential along with a small grey water tank. Issue would be plumbing a filler neck/cap and without having inspected closely, believe it can be cut into the panel at the rear below the rear doors.
Just an idea