I would bet first your battery is discharged, but an original univolt that old should not be trusted.
I would check the voltage, then plug in to shore power, and check the voltage again. That will be quick checks of the battery and the univolt.
"Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace." James Madison
'08 34' Classic SO, '12 Ram 2500 4X4, 6.7L Cummins.