AC shuts off after only running a few minutes

Your Tstat should be controlled by the front AC control board. There are 2 power 12 volt wires to that control board. Make sure you have 12 volts positive and negative on the RED and BLACK wire feeding the front control board. This power wire set controls ALL functions of all 3 control boards. The 2nd AC control board is connected to the front and the 3rd control board to the Middle(2nd) by a phone type communication cable. So, your problem is either the Front AC control board or the telephone type communication cable between the Wall Tstat and the front Control board. It is extremely rare for the control board to be at fault on these type problems. At the front AC, you have a harness that has 2 phone type connections on 2 pigtails. Those pigtails are black. The wires running to the middle AC and the wall tstat will either be white or gray. Check the connections on the phone type modular plugs, disconnect them and look for corrosion. Your year and brand should have lo profile Penguin models. These are susceptible to excess humidity where these wires are and cause corrosion. The fix if you find rust/corrosion on the modular plugs is to cut the modular plugs OFF and install new modular plugs. If there is NO corrosion then the problem will be a defective phone type cable from any off the AC units and the wall Tstat. ONE shorted cable in any of these cables can cause a problem. The usual test is to take the wall tstat up to the front AC Modular cable connections and plug it directly into the front AC board with the Middle supply cable disconnected at the front AC and see if the front functions. If it does, you then leave the 2nd cable disconnected and reconnect the wall cable at the front board and reconnect the tstat at the wall and see if it still functions correctly. If it DOES your problem is isolated to the 2 rear AC unit control cables. If it does NOT, then your problem is between the front AC and the wall tstat on that cable. Doug

Your thermostat is powered with 12v DC.
Had the same type of problem on my country coach it was a bad connection in the wiring harness going to the thermostat. My thermostat would go blank just like yours when I had several lights in use connected to the same harness. It could be a loose ground wire. Mine turned out to be a harness running next to the refrigerator and the heat had caused the connection to fail.