We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Dave
saying something like:

I have just been watching them and a thought crossed my mind.

How do they stop the diesel from going solid in the tanks and pipes?

Lots of anti-waxing additive and particular attention paid to low water
content of the fuel. In extremis it isn't unusual to have heated tanks
and lines.