more like nearly dead, so maybe your gauge is off?

as for adding in the golf cart batteries and the agm's

i wouldn't attempt it unless they were seperate banks to switch the
inverter to.

sounds like you got other issues to resolve first, before adding in batteries

Don't attempt to mix batteries in a string. You can parallel mixed strings without too much problem but each string must contain identical batteries or you will have to baby sit them constantly and do individual equalising ( if you know what you are doing).

i would get a decent hydrometer one of the temp compensated ones

draw up a chart listing each and every cell in every battery
and take specific gravity readings and chart each cell after they have
been fully charged (or what you think is fully charged)

they should top out at about 1.260-1.270 although some manufactures run higher
to get higher amp/hr capacity out of a smaller battery by sacrificing lifespan.

whatever the case, you will likely find a few cells lagging behind.

take the offending batteries out of the string and independantly equalize them
at whatever voltage the manufacture suggests,, it might be 15vdc or higher, at very low amps,, and the battery will boil, it will get warm (not hot, if it gets hot, let it cool and then hit her again) keep after it until the specific gravity doesn't rise anymore for an hour.

then let it sit and cool for 24hours and retest the spec gravity
if it is off by more than .005 the battery has problems and will likely get out of
whack soon again causing more problems.

i am thinking that the string should be at or above 50.8 volts at rest after 24 hours, at 77 degree's F and be fully charged.

48volts and they are likely down to about the last 20% of remaining capacity, in which case it will take forever to recharge unless you got some good charger capacity, even then i would expect many hours or engine run time.

best i can suggest,
sooner you get ontop of this issue the better off you will be, waiting only makes recovery more difficult if not impossible.