Battery voltage drops

[Falcon]

2019 3791RD, 20th anniv. edition: While boondocking, we experienced a slow loss of voltage over days. Each morning the PV panel seemed to bolster things but each day it was less. I have two Honda 2000i's, paralleled, I'm told is worth 26.2 amps (one AC unit and the microwave trips them out). I ran these for hours without any increase in battery voltage. I hooked up the trailer wire to the truck and ran it for an hour, no increase. But days earlier, when hooked up to shore power at a park, the battery voltage went right up to 13.9. But I'm reading this from a digital gauge on the outside of the control panel cabinet. I'm not even sure what I'm reading. Is this the real voltage from the batteries? Is this how much the PV panel is putting out? is it something else all together? It has a logo on it that matches the PV panel system logo. When voltage runs low, there is a curious sound in between the ODS pass-through storage front panel and the battery compartment. I recorded it on a phone video but can't attach it (it's an mpeg). It's a sharp click, then a crackling sound, then a "boing" like you plucked a string. It's about 3 seconds for the three sounds, then repeats about 6 seconds later. Any help or experience is most welcomed.