Now would be a great time to think about an upgrade to your electrical system, Specifically the install of a Progressive Industries HW-30C. WHY?
Well the next time your In-Laws borrow the trailer, Promissing faithfully not to fry it again, and .. plug into another 220 volt Dryer/Et-al, outlet.. The HW-30 C will protect you.. DO NOT TELL THEM HOW TO BYPASS IT.
Now, the list of possible fried products incledes not only the Fridge A/C Element, though in truth it may have survived, Heating elementes are among the last to blow when this happens.. but the control board (the 120 volt sense circuit) which is much more fragile.
Same for the water heater.
Air conditioner, both control and motors
TV, Radios (if not 12 volt) Microwave
The converter
And all other 120 volt "Stuff".
Check it all out,, Have fun,, Hand him the bill.. DUCK.
But put in that HW-30 C
On a positive note: Some modern electronics uses a newer type of power supply (Switching) and some of those can take 240 volt no problem, this includes TV, Radio, Converter, but likely not the microwave or Fridge and water heater. Sorry about those.