We used one of these when we had our twin axle caravan, so about the same floor space and cubic m as your average MH. http://www.appliancesdirect.co.uk/p/exp09hn1wi/electrolux-exp09hn1wi-air-conditioner
It worked very well at keeping the dog cool when we had a very hot spell in the UK in 2013. A few things to note though:
You'll need to vent the exhaust through an open window. The end of the 4" exhaust hose terminated in a rectangular piece which goes through the open window but means the window has to be open quite a way. Then you have to contend with or block the "open" section next to the outlet so hot air doesn't come back in. Security is also then compromised as you may want to run the a/c when out and about but the window will be open!
Most portable a/c units only direct the cooled air in one direction so you can experience overcooled parts of the hab area whilst still having warm areas.
Although the one we had was 9000btu, it didn't feel that good at cooling unless it was run flat out and then it became very noisy in the 'van. Certainly not as quiet as a roof mounted unit. Found it impossible to sleep at night with the a/c on, again due to the noise.
You need to locate the main unit as close to a window as possible to minimise the length of exhaust hose. According to Electrolux, extending the hose can reduce the efficiency of the unit and I'd concur with this, having tried to tidy up the exhaust hose and routing it outside the 'van with extra ducting.
Ours was a big 'van but the a/c unit still got in our way. It was on wheels so we could move it around when not in use. Good job as it was bloody heavy.
It worked well (apart from the noise) if we kept the blinds closed before any sun got into the 'van or the 'van started to heat up. This meant we spent most of the day in the dark when in the 'van lol.
If floor space and weight aren't an issue, then a free standing a/c unit might suit you. We "lost" ours in a dispute with the owner of a seasonal site in 2014 but that's another story lol. Suffice to say we haven't replaced it after using it just once in our Kontiki tag and are now erring towards a roof unit. More money but many more benefits, space being one of them.