Where did all the power go?

Where did all the power go?

1995 Electric E-Z-Go Golf Cart, 36 volt. All 6 Batteries reading 6+volts, power gauge on dash reads Full. Engage key, place in Forward and depress accelerator...result= Cart moves slowly for about 1 mile, power gauge on dash decreased to low...BUT ...I immediately re-verify each battery and all 6 still show 6+ volts... could this be a problem with the Solenoid ?