The longest trip we have done so far is 3 weeks which we have done twice without staying in a motel once. For us the SMB is positively luxurious, previous to having the SMB we camped with a teardrop trailer pulled by my Jeep Wrangler and the longest trip we took in that was 2 months traveling through Mexico.
I can't speak for Central and South America but in Mexico the Diesel fuel is all low sulfer diesel (500 Parts per million) not ultra low sulfer diesel (15ppm) like we have in the US. Even though the newer SMB's say they require ULSD they are the exact same engine as the earlier models and should run LSD just fine. The issue with newer diesels designed for ULSD is the Diesel Particulate Filter (DPF) may clog if used with LSD. Since the 6.0 is not designed to use and the vans don't have a DPF the LSD should work just fine. The one advantage to the diesel is diesels have a much better fuel filtering system than gas engines, especially the 6.0psd. Changing the filter, especially the primary one is a piece of cake. You would need to carry extra filters and I would recommend changing one yourself before you go so you are familiar with the procedures and have the correct tools with you.
The gas in Mexico tends to be a little lower octane than here in the US, at least my Jeep though so because it never pinged here but did on Mexican gas. I did buy the cheap stuff though and like here they had different grades.
Both diesel and gas were available at almost all gas stations. There were a few that didn't have diesel pumps they were definitely the exception.
Whichever you choose following my 4th rule of Mexican driving is advisable, always start looking for fuel when you reach 1/2 tank. Sometimes gas stations are inexplicably be closed our out of order or there are a group of guys hanging around that looked like they are up to no good. If he stations are closed there always seems to be someone selling gas out of drums in the back of a pickup truck, we never had to buy from them but I am sure they add a hefty convenience fee.