The first step should be determining if your ACs are actually doing their jobs. A laser thermometer is not a good choice for this step, they are meant to read surface temperatures, not air temps. Get a digital BBQ thermometer if you don't already have one. Stick the probe into a supply vent closest to the unit and record the air temp. Drop the grille and remove filter media in a return, stick the probe into the return duct. Record this temperature also. Rinse and repeat the operation on the second unit. I should have stated earlier; make sure the units have been running for 15-30 minutes before measuring. You should see temperature differences between 18-22 degrees between the supply and return ducts, if you're in that range, the AC is likely doing it's job. The temperature difference represents the total amount of heat the AC is removing.
The best AC ever made is totally dependent on the air distribution system to provide comfort. Does your rig have the "Blade" system vents? Even though Keystone invented them and had them patented, I believe they turned out to be ineffective and have now moved away from them. If you do have the Blade system, remove them and take them to the nearest dumpster or if you're environmentally conscious the nearest recycling center.
Seriously, the first place to address is the bathroom. On rigs with the bath at the top of stairs, like yours and mine, with the placement being in an area of highest pressure in the duct system, they typically get way too much air flow. Remove the vent in the bath and using masking tape, cover the outlet with tape. You can start by cutting some slits in the tape to provide some airflow. The idea is to force some of this air into other areas of the rig. I believe you'll find it takes very little air to cool that very small area. If you find this helps, I'll recommend an adjustable vent later. Obviously, the tape allows you to experiment with how much air is actually needed, you can always cut more slits or add more tape.
Using your BBQ thermometer, slowly walk around the coach noting temperatures in different areas. You are looking for a difference of more than +/- 2 degrees. It would be a good idea to lower the window blinds for this test; reduce as much solar gain as possible. If you find areas much warmer than others, just remove the vent closest to that area temporarily. You'll have to be patient, it may take a full day to see if this improves anything; at that time, rinse and repeat the process to see if it helped. If this turns out to be the issue, there are options; right now just trying to pinpoint the problem.
You mentioned the filters in the returns, are you using the thin foam media like the OEM supplied? That stuff is restrictive when new and clean and will load up quickly only making thing worse. I like the polyester "horsehair" media like this:
Amazon.com
The returns are the most critical ducts in the system; the unit can only supply as much air as it can take in.
You can find similar media at hardware and big box stores, but it's normally much thicker and may not work in an RV application, get the lowest MERV number you can find if they even list it. I found some that came on a roll and cut them out with scissors using the foam as a template. This stuff is durable enough that I put mine in the washing machine on delicate cycle. Don't be concerned about air filtration in your coach, this filters job is to keep large particles out of the evaporator coil. If you feel you need a little more filtration, just spray them with Pledge when you clean them.
Should you decide to try this method, pleas post your results and maybe we can provide some more ideas at that time.
