[BB_TX]

Using voltmeter check voltage across the two poles of your main 50 amp breaker. Should read 240 vac. If not, then something wrong upstream from there.
If you measure 240 vac across the main breaker, then check each main breaker pole to neutral. If 240 vac across the breaker but low voltage on each leg to neutral, then you have a bad neutral.
Also measure the voltage from each main breaker pole to ground. That should also measure 120 vac. Good voltage to ground but low to neutral confirms bad neutral.