[JRTJH]

True "deep cycle" batteries are a "breed all their own"....

Most "hybrid" batteries are called "deep cycle" but really aren't. Most are "marine/RV deep cycle" which means they are required to provide both "cranking capacity" (high amp draw) and "continuous capacity" (slow steady discharge). The "hybrid" will always have CCA or Cold Cranking Amps listed on it. The "deep cycle" battery will not have that designation.

What makes them different? The "true deep cycle" battery has thicker plates which can withstand repeated deeper discharges without failure. The "hybrid" has thinner plates so it can deliver higher "cranking amps" (not needed in a true deep cycle battery) but because of the thinner plates, is more subject to damage and failure when discharged deeply.

The batteries in my truck are 5 years old (Motorcraft) and I just replaced the battery in my DW's Escape (2008 model) last year, after 10 years of use (also Motorcraft). I have not experienced the early failure that you say your fleet is experiencing.

As for "true deep cycle" 12 volt batteries, they are available, usually cost slightly more than two 6 volt batteries of similar 20 amp discharge ratings. That's why most people (IMHO) buy 6 volt batteries: Easier to handle due to weight and cost a bit less.