I have a Coleman Powermate 800 watt power inverter(12 volt to 110 AC).
The red fault light comes on
along with the green light. (this as I understand it means there is
not enough voltage to run things is this coreect?)
Sometimes it worked sometimes it didn't now it doesn't at all. I
checked the fuses. I checked it on two different fully charge
batteries. What is wrong?

Does the red fault light come on when you have no load connected?

If the red light comes on with no load connected, you have something
wrong with the inverter.  If the red light comes on when you connect
the load, the load is probably wanting more than 800 watts.  You will
need a larger inverter, or a smaller load.