Hello Martin,
I have been involved in testing all sorts heating and ventilation including air conditioning systems for caravans, and it is as well to be aware of some fundamentals here, otherwise you might have unrealisable expectations of equipment performance.
Do not get confused with the evaporative air coolers and proper AC units. The evaporative unit need to be charged with water, and they work by air passing over a moist wick. As the air passes, it picks up moisture by giving up some of its heat to evaporate the water from the wick. The air emerges cooler but also laden with moisture. The down side is they only drop the temperature by a small amount but they radically increase the humidity of the air, and that can be as uncomfortable as being too hot. The danger is the wicks are perfect breeding grounds for bacteria including legionella.
A true AC unit will have two heat exchanger coils, and the best units segregate the air flows across them to keep then totally separate. The evaporator coil drags heat out of the air and is the coil that services the living area, The second coils is the condensor which is used to get rid of the heat to the outside. Ideally the condenser coil should have both the intake and outlet air flows for the coil ducted to the outside. You may need to provide additional external fittings to allow this, otherwise if the exhaust is discharged back into the living space, you will actually be adding more heat.
The smaller portable AC units are typical rated at about 1.2kW. Anything much bigger and you may have problems with the sites electrical supply limits.
To put the power of this into context, most AC systems in cars are rated at about 5kW, so consider that the volume of a car is no more than about 5M3 and a typical 2 berth caravan will be 20M3 so you are using something with only 25% of the cooling capacity in at least 4 times the volume, so it will be about 1/16 as effective as your car.
So it has to be used carefully to maximise the benefits. Do not expect it pull down the temperature by a significant amount throughout the whole caravan, but aim at your seating area and this may provide the most comfortable strategy.
I have used a portable unit in a test situation, and whilst it is not quite as effective as an in-situ unit it did make a comforting difference.
Not only was it significantly cheaper than a puka caravan model, as you have already mentions its uses are not limited to the caravan.