Good idea. I've been in a couple of campers that have a warning buzzer when hooked up if ignition on and quite liked that idea so I built a circuit to do the same.
Maybe bit over the top, but because I have a
Victron Cerbo fitted, I could use the Node Red feature to setup a programmable relay to drive a buzzer when it detects AC in, but only when the Ignition is on.
Not as precise as yours as if there is no live AC the buzzer will not be live even if EHU lead is plugged in, but on the flip side I can use the same feed to have a AC socket that I can chose to be active only on live EHU if I want (useful for an AC Heater).