I normally can trouble shoot and fix every thing with my truck and every ones else's however this is something I don't really know that much about so I am going post this hoping some one with more knowledge them me can shed some light.

I have a 91 jimmy 4.3 with the digital dash, The fuel gauge is stuck on full and will not move.
The fuel pump is working fine and i have good fuel pressure.

Is there any thing besides replacing the sending unit I can check for that would make the fuel gauge read full?

another small issue is the voltage meter reads 10 volts when the heater is on or when i have the headlights on, I have a new battery and with a load tester its 12.8 volts at 990 cca.

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If the wire from the fuel level sending unit to the gauge cluster has been broken/disconnected, it can cause the gauge to read full all of the time. The test for this is to disconnect the wire at the fuel tank and ground it. If the gauge then reads empty, then the wire is fine and the sending unit is bad. If the gauge reads full, then you have a break in the wire that needs to be repaired. I believe this wire is pink both at the cluster connection and at the fuel pump. The fuel level sending unit should have around a 90ohm resistance when full, 35ohm at half tank, and zero ohm (continuity to ground) when empty if you want to test the sending unit.

As far as your voltage gauge goes, it sounds like the current from the fan and/or headlights is higher than what your alternator is putting out. I would have a charging system test performed (most autoparts stores do it for free) to see if it is an alternator problem. If that checks out fine, then here is the diagnostic steps for the cluster:

 

This weekend I will get in the garage and start testing the wires.
I assumed it was wires or the unit its self, I did not know what the process was.

Thank you for taking the time to do a good write up, I think you explained it well.

After I correct the issue I will reply with the result.
Thanks again.