Look for a junction box between the genset and the rig's wiring to transfer switch. This is commonly at the rig's end of the flexible conduit.
You could tap at this location. You would need a circuit breaker to protect the receptacle then run to EVSE receptacle.
This should be a 120/240 four wire circuit.
Bud
USAF Retired
Pace Arrow
2003 Chev Ice Road Tracker