It's possible to power the RV by wiring a plug (pigtail or not) to the back of the shore power inlet plug/wiring. The issue is that if he does that, when the electriity flows through his plug/pigtail plug, it goes both ways at the splice. It flows into the trailer, but unfortunately, it also flows to the male pins in the OEM shore power plug.
Without either a manual "toggle switch" or a transfer switch, he can't choose which way the battery/generator power is going to flow. It will flow both into the trailer and out of the trailer from his "behind the shore power splice.
I think you're talking about adding a complete plug to the input wiring behind the shore power plug on the trailer skin??? That way, he can "unplug the newly installed plug making the outside shore power outlet "dead" and then plug the other end of his newly installed plug into the generator/battery bank.