First thing to check is to see if the ACs are actually removing heat. A temp gun isn't the best choice for this, they are made to read surface temps, not air temps. I prefer a digital BBQ thermometer. Stick the probe in a supply vent closest to the unit, next stick the probe into the return. When you check the return, remove the grille and filter and stick the probe into the duct as far as possible without touching the insulation. Record your measurements and repeat with the other 2 units. If the units have been running for 15 minutes or more, you should see between 18-22 degree temperature differences between supply and returns. If you are seeing acceptable temp differences, this is a pretty good indication the ACs are doing their job.
If the ACs are doing their jobs, the issue is likely in the air distribution. Using your BBQ thermometer, walk around the camper spot checking temps in different areas. Are some areas abnormally warmer or cooler than others? A good rule of thumb is anything more than +/- 2 degrees. A usual suspect area is the bathroom, it's a small room and not occupied for very long periods of time usually. I see yours has a sliding door that may or may not be left open; those of us with the infamous pivot doors it's not optional to leave them open. Remove the diffuser (grille) on the bathroom vent and using masking tape, cover the opening. Start by punching or slicing some small holes in the masking tape. I believe you'll find it takes very little air supply to that room, and any visits that take some time the exhaust fan is likely running anyway. Don't be concerned, this is just an experiment, we'll address it later. If you find spots that are abnormally warmer than others, remove the celling diffusers in that area and just leave the holes open. Some just snap into a trim ring, others have a couple of screws; either case it's very easy. It won't change quickly, may take a couple of hours to notice any difference. Go back through and check temps in the same locations and compare to original ones.
If you have anything to check it with, check the humidity also. I'm sure living in Houston, you are painfully aware of humidity and high dew points. My experience is that RV air conditioners do a really good job of removing humidity. Partially because the total air flow of the blower is a little lower than you would normally set a residential unit of that size for.
I found a very sloppy job of cutting away the ceiling panels under the return grilles on my rig. If you can see any of the ceiling material under the trim ring not cut out, it's restricting air flow. I believe Keystone has moved on from their heralded "Blade" system, but normally use some very restrictive supply vents. In a system with that little airflow to begin with, just a square inch or two here and there of restriction can make a big difference in air delivery. Another thing to check for is are there any ceiling supply vents with angled blades pointing towards a return? It's called an air short circuit and simply means a lot of your cooled air is just blowing straight into the return rather than the space. rotate them away as much as possible from the return.
You stated that your rig has 3 units; normally the 3 unit rigs have a non-ducted in the center. Is this your case? If so, that unit should be honking. There are no restrictions on that one.
If you could do these tests and let us know the results there are other options to consider. It would be best if you took the outdoor temperature and humidity levels at same time too.