Steve & Raz - is there a bond between the converter case & the trailer frame in your trailers? If not, there should be.
While there should not be a bond between the neutral
& the trailer frame, the enclosure must be bonded to the chassis (frame).
Here is NEC 551.20 (C):
"(C). Bonding Voltage Converter Enclosures - The noncurrent-
carrying metal enclosure of the voltage converter
shall be connected to the frame of the vehicle with a minimum
8 AWG copper conductor. The voltage converter shall
be provided with a separate chassis bonding conductor that
shall not be used as a current-carrying conductor."
Article 551.56 covers the 120V side of things -
"551.46 Bonding of Non-Current-Carrying Metal Parts.
(A) Required Bonding. All exposed non-current-carrying
metal parts that may become energized shall be effectively
bonded to the grounding terminal or enclosure of the distribution
Bonding Chassis. *That is suppose to be a "B" not a smiley*
A bonding conductor shall be connected
between any distribution panelboard and an accessible
terminal on the chassis. Aluminum or copper-clad aluminum
conductors shall not be used for bonding if such conductors or
their terminals are exposed to corrosive elements.
Exception: Any recreational vehicle that employs a unitized
metal chassis-frame construction to which the distribution
panelboard is securely fastened with a bolt(s) and
nut(s) or by welding or riveting shall be considered to be
There is more that covers things like gas lines, metal roofs, furnace
& ducts, etc but the end result is that the ground system of the trailer must be connected to the chassis (frame).