I know you want "The Answer" but you could have quite a few things going on that could cause what you are experienceing.
Lots of good information here about battery life and maintenance (even if they are "no maintenance" type.
Best battery faq ever
Running a battery dead (less than 10 volts) is the absolute worse thing you can do for many reasons. Ruining it permenantly is only one of several bad outcomes.
An OEM Dual Purpose deep cycle battery has about 75 amp hours of capacity and may take days using the onboard charger (powered by your generator) to completely refill. The reason is that the charging is done in stages to avoid boiling the electrolyte out of your battery. While the first stage may put in about half of what you used in an hour or two; the second and third stages (where the amps are reduced to allow complete saturation of the plates) can take a couple of days.
By running the battery dead the first night put you WAY behind the "power curve" and you never did catch up.
Th guage will show "full" ONLY if you are using the liquid tank buttons (fresh, black, or Gray). When you use those lights for the battery, it will read "CHARGING" not FULL. (See the attached manual)
Putting it away partially charged is also a bad idea. Even with the disconnect you will lose power through a process normal for batteries called "self discharge" through the internal resistance of the battery. It will take a few weeks with a disconnect even if it is put away fully charged. That is why folks put trickle chargers (battery minders) or solar panels on their batteries when in storage.
The OEM does not configure their campers "out of the box" for extended dry camping. The battery is "sized" to provide emergency brakes and a night (only) of camping with about 4 hours of furnace use. Most folks who dry camp do extensive modifications to their 12 volt system to make the camper more "dry camping" friendly.