Would two people standing on the ground touching the truck frame qualify as two faults?
It's possible for two people to be the two fault paths, yes, but not simply if two people are touching between the truck frame and ground - they would be more like two parallel conductors of the same fault - i.e. two neutral wires.
For their to be a life-safety issue, a human(s) must complete the circuit - i.e. one hand touching a hot wire, the other touching either a ground that is ground -> neutral bonded or a neutral wire.
In the US electrical system, because the neutral must be bonded to ground, we basically make the earth a part of the electrical system. This has pros and cons. One of the downsides is that a person standing on conductive ground - i.e. a wet concrete slab or soil with bare feet is already a part of the circuit, and touching a hot wire will cause a shock.
In the mobile scenario, there's no neutral -> ground bond, so one person can be standing barefoot on wet ground and touch the hot wire on the trailer with no risk, as there's no completed circuit path from the ground to the neutral of the trailer. Hypothetically person A could be earth grounded and touching the hot wire and person B could be earth-grounded and touching the neutral wire, which would indeed complete the circuit, albeit with a fairly high impedance.
If you can ensure that your electrical environment will always be floating - as with a small single-circuit generator like a Honda EU2000, you can make an argument that not bonding is better, but this becomes problematic when the size of your system and the potential for current sources from multiple places grows.