I put a 5300 btu window air in ours and it has held up great. I reinforced it to the wall probably more than one should.

It lacks in fast cooling, but its just fine for us. Our LQ is 15 1/2 feet open box length plus the neck over. We just let it run constant instead of on/off cycles.

I got the window air for $40 and it came with a remote. It cost me the price of 1 2x4 and some screws to mount it. The roof air, even to find one used off of a pop-up was in the $400 range, plus having to reinforce the roof just wasn't worth the price differential. I would also have had to lose my roof vent/fan which I like too much.

You may also want to pay attention to your amp service you have running to the trailer. Mine is only 15 amp service (came out of a truck camper--soon changing to 20 amps) so running a large unit really wasn't a safe option.

I went with a portable air-conditioner.  When I priced a roof air-conditioner, it would have cost me around $1,200 to buy the air-conditioner and to have it wired and installed.  With the portable air conditioner, I shopped e-bay at the end of the season and was able to get a new 5000 BTU for around $300 - $325.  Cheaper than my local stores even with shipping.  The air-conditioner has a thermostat, timer & remote.  There's several different sizes and BTU's so I measured the area and found one that fits well and it doesn't take up much space.  Another thing I made sure of is that it was one I could carry myself because some a very heavy and mine weighs around 50 lbs. or under.

Then I went to my local glass/repair company and had them put me in a hole in the window to vent out the duct.  The air conditioner came with the parts so I can leave the piece to attach the hose to in the window with the cover or I can take it out.  The gentleman that put it in put another piece of plexi-glass next to the vent hole that slides so if I take out the attachment piece, I can slide it over and it covers the hole and is sealed.  I used the window figuring the placement and figured it was the easiest but a person could vent out anywhere.

I have a 7ft on the short wall living quarters and the last time I cooled it down, I had it down to 68 degrees.  I did find you want to start it before it gets real hot like in the morning or it will take a longer time to get the trailer to cool down.

I'm happy with what I have and I was able to do it for around $400.00

I should add to my last post that I did not cut a hole in the trailer skin. I know some people that put the window air under the goosneck. I put mine in the back wall facing the gooseneck. It's up high and out of the way. There's plenty of airflow through the windows and into the horse area..it does the job.

I missed a great deal (before it started getting hot) on a portable air unit for $75. That's the route I wanted to go originally. However, I like this choice just as much and for $40 couldn't beat it.

These are about as cheap as you'll find for a roof mount... http://www.pplmotorhomes.com/parts/duo-therm-rv-air-conditioner-parts.htm

The Coleman units are more and the Carrier is about twice the price of a Duo-therm. RV dealer will charge another $400.00+ to install and wire unit to a breaker box.