A dehumidifier is, IMHO, the correct choice. For those why might "still be on the fence" with their choice, consider that with a dehumidifier, you remove moisture from the air at any temperature. With an air conditioner, you only remove moisture from the air when the compressor is running. The compressor only runs when the thermostat calls for cooling, so you lose any "dehumidification" when the outside temperature falls below the thermostat setting. So, for an example, if you set your thermostat to 65F, you'll only be dehumidifying the air in the camper when the temperature is above that temperature. If the temperature falls below that setting, your air conditioner compressor will not run until the temperature rises above that setting. So conceivably, you might go for weeks on end with the air conditioner not removing any moisture from the air. And, if you set the thermostat to the lowest setting, chances are you'll risk the A/C freezing up on a cool, wet night. Once that happens, it may never recover by thawing the coils. And, to top all that off, the filter (a thin foam sheet in most A/C's) will get dirty in short order (usually in a week or two) and after that, the interior will start to smell "musty" and you'll have reduced air flow. Or, you could plug the filter completely, in which case, you may find it "pulled away in the corners" and all that dust is going directly into the coils, making for a "frustrating cleaning job" before the A/C works again.
What I'm saying, in a nutshell, is that the A/C is a good dehumidifier if you're present and the temperature is right for the A/C to run. It's not a good choice to be left alone for weeks or months on end while the trailer is in storage for many reasons, some of which I've listed above..... YMMV
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS