97 Cummins won't stay charged
 

Just bought my first diesel truck. 97 12v Cummins. First off the tach and Speedo don't work.replaced the 90amp alternator and belt, and two red to optima batteries, I can go a full day driving just fine next day halfway through the batteries are dead. Charge em up their fine. Did a power check on the batteries while running batteries aren't getting a charge from alternator. % tried the ford part to truck the alternator into charging, didn't work still slowly drains. Does anyone know what in the world is going on. This is mydaily driver and need it reliable asap!