THREE very strong suggestions:

1)Drive a diesel powered truck. Diesel does not burn the way petrol does.
2)Store an extinguisher on the OUTSIDE of the truck.
3) Give serious consideration to storing everything that is vital in a flood and fire-rated container. So that if the truck does drown/burn you have a much better chance of recovering your gear.

Sorry: Make that FOUR suggestions:

4) Attend a fire fighting course and learn how to use an extinguisher properly. I have actually watched a bloke try to put an engine fire out and fail, because he did not use it properly. He kept on using it in short bursts and ran out of suppressant. If he had given it the whole load in one substained burst, he would still have a car.