I just bought a progressive industries EMS and am in the process of installation. Here is what it says in their installation instructions. Quote
"• RV wiring is different than the wiring found in homes; the
neutral and ground conductors are isolated in the RV, unlike in a
home where they are tied together at the service panel. The
reason is: homes have a bonded ground system, whereas RV’s
do not. Therefore, never bond the neutral and ground together
for any reason. This will create a ground fault condition, and
may result in electric shock and/or fire hazard."
I think four conductor #6 AWG is what is required!!
P.S. I bought a meter of 6/4 at home depot