Ok I am going to assume this is a motor home.. Your mistake is 45 minutes.. More on that later
On a motor home less someone has messed with the wiring (Very common) there are two sets of batteries. Engine and HOUSE.. And a device called an "Isolator" that keeps them apart if the engine or other charging system are not running.. Isolators come on 2,2 types
One is a bi-directional (I have that, Either system (House converter or Engine alternator) Charges ALL batteries
And the second and both points are one way.. Eitehr a set of doides or a solenoid that allow ENGINE to charge house only
Takes SIX hours to fully charge 'em minimum, (The 45 min miostake)
Now. for some reaon.. Some folks bypass the isolator so you have just one bank ENgine and house combined.. I"d fix that as it's meant to prevent just this problem
Also one time M& isolator.. Stuck. (Just once) and did not isolate
How I fixed
I used a small jump start battery (Actually a garden tractor job) to fire up the GENERATOR. and a short time later at 80 amp;s converter output plus the 100 amp; back up charger.. Everythign worked.
