An easy, but electrically inefficient, solution is to add another equal resistance in series with the 120 volt fridge
If the US market fridge
element heat output is 120 watts when powered by 120 volts input, the current is 1 ampere (watts equals volts times amps). Calculating the resistance (ohms equals volts divided by amps) for that theoretical heat element works out to 120 ohms.
Connect that 120 ohm resistance 120 volt heater to the 240 volt supply in the camper and the doubled voltage will push double the amperage through the heater element. Double voltage at double amps is 4 times the heat output.
Adding another 120 ohms resistance either before or after the US heater's 120 ohms resistance will cut the apparent voltage at the heater to 120.
The inefficiency is that the added 120 ohm resistance is also a heat source and is also producing 120 watts of heat. That additional heat will need to be kept away from the ammonia tubes.
Be aware that the European 240 volt AC mains is different than the North American 240 volt AC in that the European supply is zero volt, +240 volt, and earth/ground, whereas North American 240 volt supply is +120 volt, -120 volt, and earth/ground. The potential is 240 volts for both (European zero to +240, or North American -120 to +120), but the one “hot” lead, and two neutral / ground leads in Europe versus two hot leads, one ground in NA, may be another issue.
Unless you convert all the camper's AC appliances (roof air conditioner, microwave
, and such) to North American 120 volt, you'll still be using the camper's 240 volt system, probably supplied by a 12 volt DC to 240 volt AC inverter installed in the camper in Europe. You'd not be plugging into a North American campground's 120 volt power source anyway.
Since you need the camper's 12 volt DC circuit to run the other 240 volt AC items, just run the new fridge on 12 volts DC. Buy a large capacity battery
charger, plug that into the campground's 120 volt AC, connect to the camper's battery
to keep it charged, and use the camper's battery
to run the on-board 240 volt inverter.
Not elegant, but workable.
Jon MB, the pretend electrician of the pair