Hmmm. That does give one pause.

[theory mode, probably only tangentially related to reality]
I suppose the frame would provide a near 0-ohm ground plane, unless it breaks (!), so in theory it should be fine. All that steel should conduct much better than even two 1/0 cables... assuming, as you say, that the connection is the cleanest one on the truck and remains so. Unless the ground plane was broken, it seems unlikely that any current would 'look' for another path.

[contrarian mode]
On the other hand, hundreds of amps can cause odd behavior even in the presence of one ohm.

Any comment from others?