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.
"The time is always right, to do what's right." MLK
'08 34' Classic SO, '12 Ram 2500 4X4, 6.7L Cummins.