I live in Texas as well and I can tell you that a heat strip is of no use.  Get a small ceramic heater from Wal-Mart and you will be cozy warm.  I use one in my 6' SW and it is fine.  can't help you on the AC.  Whatever they put in mine when I bought it works well even in Texas.  Just the standard roof air by Carrier, I think.

I agree the heat strips are worthless.   Don't waste your money.

How big a generator do you have?  Make sure it will run what ever A/C you buy.  The two LQ trailers I have had both had 18,500 btu units.  So I figured they were pretty much the standard size for horse trailer LQ.

I never use my A/C camping, since I primative camp in the mountains and it's usually pretty cool there. I never go to fairgrounds or horse show, so there is no sweltering in a parking lot for me. The only time I turn on the A/C is at home when I am loading or unpacking the trailer for trip. And then it seem I can't get enough cold air. But then again we are opening and closing the door a lot while we clean out the trailer. So my advice is go the bigger unit.

I was in a friend's trailer, that was outfitted at the Integrity dealer in Texas if that matters, and her AC fans blow out from a circular opening so less air is forced right out onto the bed area.  I would like that at night and I bet it keeps the trailer a more uniform temperature.  My AC has more of a square area for the cold air to either blow onto the bed or towards the slant wall.  Does that make sense? 

And I used my heat strip and it takes the edge off the cold, which is all I needed that evening but will take a plug in if it ever is cold when I use the trailer.

All AC units are good.  We have a carrier and I just spent about a total time of 8 Hrs bending the fins back in place because of a hail storm.  I would look a several units that have covers over the fins.  

I chose a Carrier unit without a heat strip.  The ceramic heater is much less noisy that the A/C circulating fan.

Added benefit, when the hook-up allows, I'll use two heaters.

Another benefit with the ceraminc heaters is you can adjust the temp so it will turn off and on, but they are very quiet.  They also don't blow on you like the AC units.

I did find a control on the AC unit where the vents will move up & down, so it's not a constant steady blow of cold air on you.

Whats the average lifespan of an AC unit?

Because they sit on top in the hot sun, do they deteriorate over time if you do not keep your trailer in an RV garage, barn shed or shelter?