What causes trucks to burn?

I know the term and yes you might get banned for referring to an old testament culture and mega static electricity bursts from storms. Naughty you.

Dang. I might get kicked now. Well I probably need to be for that one.

 

Some of y'all hit on it with flash point and vapors.

petroleum distillates will only burn as a vapor. The temperature point at which they vaporize is their flashpoint. the air still needs to be saturated with enough fuel vapor to ignite. This involves exposed surface area of the fuel which affects it's ability to vaporize.
The reason you can drop a match into a bucket of No.2 fuel oil and it will douse the match but fill a sprayer with the same fuel and spray a lit flame and you'll have a mini flamethrower. The atomized droplets create more surface area resulting in faster vaporization, saturating the air enough to reach flashpoint at a lower temperature than if just left in a static pool to evaporate naturally.
Heavier fuels have lower flash points like oil for instance. Lighter fuels like gasoline vaporize at lower temperatures so checking your gas tank with a flame for light is a bad idea.

Cars have plastic fuel tanks protected in the center of the vehicle. Trucks have huge saddle bag type fuel tanks made of brittle metal that cracks easily in an accident. Inadequate lubrication can result in friction causing metal to glow cherry red and start a truck on fire while it's going down the road. Multiple batteries (enough amperage to weld with) compared to cars' single battery, lots of flammable material like rubber, aluminum, and plastic with hundreds of gallons of fuel and multiple ignition sources.

 

Yep you need fuel, ignition and air to burn. 3-4 batteries, a hot jump post on the firewall, and a hot starter solenoid post are three arc points enough to start any fire.

I would make sure my batteries are bolted down, make sure the jump post has the protective cover on it and the solenoid have a protective cover on it though most are exposed they do make covers for them. A simple pipe cap would work.

Another source is where the inverter cables go through the floor. Any crunching or shifting of that metal and you have a direct short unless you have it protected well.

The fumes is the dangerous part. The more heat the more fumes. That's the whole purpose of injectors is to spray a fine mist.