What's causing my truck to run low voltage?

If it runs at 10 volts you still have a charging system issue. Does the large wire on the alternator have battery voltage to it at all times?

Yes

Do you have an external voltage regulator?

Have your new alternator tested. It may be bad and this is a fairly common occurrence with aftermarket auto parts.

Changed crappy $10 starter solenoid for $50motorcraft, took out kill switch and push starter systems i installed 7yrs ago. Still having codes and crappy running when driving. Bought code reader got 513 with several others changed every sensor ( desperately needed). Still throwing same codes focused on the 513 EEC internal fault code. Took out EEC bottom of mount had rust and EEC body had some on it from contact with mount. Looked at firewall and saw water paths and they flowed right to gasket (wich had a bad seal because it wasn't fastened correctly) cleaned firewall and put a seal above wear the wires go to dash and EEC as well as using same material to seal where hood and firewall meet. I replaced EEC ( old had a capacitor just rattling around in there) now the truck not only runs but runs better than ever an no longer runs rich after changing air charge temp sensor wich was clogged completely.