05 F150 check charging system light started coming on at low rpm, went off when increased rpm. then abs/brake light came on. then lost all lights,gauges on dash. then lost actual a/c. truck still running fan.

You'll need a digital voltmeter. Check voltage at battery with engine off; should be around 12 volts. Start the truck and check voltage at battery again; should be around 13.5 to 14.5 volts. If the battery is not reading @ 12 volts to start with then I would take the battery out and take it to just about any auto parts store and they should be able to diagnose if the battery is truly dead or just needs to be recharged. If the battery tests ok then look at your second voltage reading and see if it's lower than @ 13 volts. If it is then replace your alternator. The only other thing I can think of is that the belt tensioner is bad causing your serpentine belt to slip causing the alternator to be unable to charge the battery.

Had same problem , turned out I was sold an alternator with bad voltage regulator.