that inverter is rated 750 watts continuous 1500 surge. your ac probably uses somewhere around 2500 watts on start up.
you have the whole trailer wired through it , didn't it have a standard plug on it?
transfer switch doesn't handle that kind of load. should be obvious with the type wire you removed, should be much thicker.
your option minus returning it, is to put the inverter on a branch circuit.
the unit you bought is not really designed for this. this was for much smaller stuff since the unit is only really rated 750 watts, which is not even half of a single 15 amp 120v circuit (1800w)