Possible electrical damage - new to RVing

My suspicion is also there is something amiss at the transfer switch, most likely miswiring that never was an issue until both the gen and plug were energized at the same time. The two relays in the transfer switch box are (should be) both mechanically and electrically interlocked so they cannot both be engaged at the same time so a failure in that case should be pretty unlikely, pointing more toward a miswire. As has been said, you did not do anything wrong, the system is designed for both to be connected without any issues. An extreme (unlikely) example would be if you had someone in your camper in an iron lung who needed to be sure power never failed you could plug in and also have the gen simultaneously running so if the shore power failed, it would just switch over to your gen and provide uninterrupted power. Please come back and post the resolution so we can all at least learn something from your misfortune. Thanks, Gary.