Because my current truck has a capless fuel system, I never put fuel into it from a fuel can anyway, as I don't want to unwrap one of the fuel funnels needed to open the filler door on capless Ford fuel fills.
That is probably the best advice above all -- only ever fill from a fuel pump. I also make sure to watch colors of containers (yellow is diesel, red is gasoline, etc.)
As an alternative, I might see about going with a pair of Honda 2000 watt gensets with an extended run tank for each of them (which will give me 72 hours of run time on both.) This way, fuel is never transferred anywhere other than its destination, further ensuring that cross-contamination will not happen.
