Open ground on rear 120v outlet.

[MtBiker19]

No true ground exists on orbital stations, aircrafts including air balloon, helicopters, boats, or other ungrounded mobile platforms including insulated vehicles like cars.

You don't need and insulated transformer, because you are already insulated from ground.

Totally agree, aircraft, no chance for ground.
At the end of the day I'm trying to power my new Airstream safely. ...and currently that involves having the Power Watchdog be that gatekeeper. Power potentially coming from the LC ...and then to the trailers 120v sub panel, which has grounded and GFCI outlets. I just want to make sure I don't make an expensive mistake. Not sure how safe things would be to connect the trailer to the LC sans Power Watchdog.

 

[Bad1975]

@MtBiker19 it all depends on whether or not the trailer connection point has a neutral bonded connection or not? If it does it won't work, if not and you are talking about connecting the ground lug on the extension cord to earth ground then you should be fine, but that wouldn't give you neutral bonding either and things like the furnace and AC wouldn't function.

I would definitely check everything with a Multimeter first though.

 

[grandsons]

At the end of the day I'm trying to power my new Airstream safely. ...and currently that involves having the Power Watchdog be that gatekeeper. Power potentially coming from the LC ...and then to the trailers 120v sub panel, which has grounded and GFCI outlets

An Airstream on wheels has no ground reference, but that's irrelevant to GFCI operation. A GFCI doesn't need a ground wire to function — it works by detecting current imbalance between the two current-carrying conductors (phase/neutral in a home outlet, or pins 1 and 2 on your LC inverter's plug). Any leakage current not returning through the expected path trips the device, ground or no ground.

One thing worth checking on your end: make sure the GFCI type matches the waveform/symmetry of your power source (e.g., pure sine vs. modified sine from an inverter). Not all GFCIs are rated to sense properly on every waveform, so it's worth confirming compatibility with your LC inverter's output.

 

[MSINAL]

Fyi our nucamp tag travel trailer works fine with shore cable plugged into LC 120v plug (using the ground prong too). Runs the Aircon, dc convertor, and the fridge simultaneously without issue. The fan for the inverter (in LC) stays on and is loud. I didnt try the microwave and Aircon simultaneously, likely would overload the LC. Now just need to disconnect the beeper for car running and no key detected.