I believe you are talking about the type where it sits on the floor and has a hose coming out of it to vent the heat out a window? Yes, they do work, I have seen them used in computer rooms and other places. Their biggest problem though is that most of them only have a single hose, meaning that they run inside air through the "hot" side of the system and then vent it outside, meaning that outside air has to get sucked in to replace the hot air that was vented outside, thus reducing the efficiency. If you can find one with two hoses to the window, one for fresh air intake and one for hot air exhaust, it would be much better, but they seem to be quite uncommon for some reason.
I did recently run across an interesting item, do a search for "zero breeze" - it's a portable air conditioner that can actually be powered by battery for a number of hours. Would love to test drive one of those at the beach dry camping, but sounds like 5-7 hours is optimistic (not a full night), and they are quite expensive, $1000+. I would probably also have to invest in more solar panels to get the battery charged during the day.
I have seen a good many people take out a window or somehow otherwise jerry-rig a window AC unit. Obviously that works, but I personally dislike how it looks. A family member has a nearly new A-Liner that actually has a small window unit built into the side of it, that doesn't look too bad. I have seen or heard of people putting them inside a cabinet or something where they can vent outside, but have the bulk of the unit inside. Depends on how crazy you want to get. Obviously the ideal route is a rooftop unit, but they don't tend to be cheap.
Someone else mentioned heat load through the window panels and such. We were camping last summer on the Eastern Shore of VA in our 89 Sunline, one of the tan colored ones. It was hot, well into the 90's as I recall. We've had our trailer 14 years now, and it ran more that week than all of the rest put together, at least several times over. The compressor ran constant from early-mid morning until about the time we were getting into bed before it started cycling off. I actually put aluminum foil in the roof vents and door window to try to block a little of the heat...didn't help we were parked out in full sun during the day. Inside temps got up well into the 80's, but the reduced humidity made it tolerable, and still cooler than outside. So while we generally don't like to run the AC, that week it was more than welcome...
We also like to stay at the NPS campgrounds on the Outer Banks of NC, with no hookups. Not bad during the day with a breeze, but mid 70's at night with pretty much 100% humidity and no air other than a 12v fan makes me wish there was something to knock down the humidity a bit...though in a few more years when the kids are out of school, we can go back to spring/fall trips rather than summer.