All good advice.
Some other information. Deep Discharge batteries are designed to be "deeply discharged" (about 10.5 volts) and recover just fine for a "designed number of cycles." They are not designed to be discharged to zero however.
Bad things happen at zero volts. At voltages less than 6 volts (LED lights do not light) the plates get heavy deposits of "Sulphate" from the acid in the electrolyte "dropping out" of solution.
The resultant reduction of ions in the electrolyte and the covering of the plates with the deposits causes two very bad things. One, the specific gravity of the "water" in the battery approaches 1.0 and it will freeze bursting the battery case or cracking the lead plates. Two, the low conductivity of the depleted electrolyte and the "insulated" plates reduces total capacity and reduced charge rates.
A Battery De-Sulphator like this one, RV Boat Sailboat Deep Cycle Battery Desulfator / Desulphator 6V or 12V | eBay
(designed to be left on all the time) will pulse the battery continuously with a higher voltage when connected to a charging source.
This will blast any deposits that form on the plates off and put that electrolyte back into solution.
Here is another model.
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2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW