My old batteries were wally world everstarts that were labeled deep cycle, trolling,marine,RV. They are dual purpose batteries, really just a starting battery slightly more tolerant of deeper discharges. They seem to have had a fully charged resting voltage of 12.8.
One of those batteries shorted a cell and became useless at month 13. The second one failed at month 32. It would read 12.8v but fall to 10.5v when any load over 0.5 amps was applied. The third is now in my engine compartment.
I replaced those 2 group 27 "deep cycles" with some true Deep Cycles.
Some Crown group 27 true Deep cycle batteries. These seem to have a fully charged resting voltage of between 12.60 and 12.65.
But the surface charge lasts much longer, and even after that has burnt off, They hold a much higher voltage overnight and with more of a load than the Everstarts ever did.
I notice that AGM batteries seem to have higher fully charged resting voltages.
Most, but not all TRUE Deep cycle batteries do not list a CCA rating, only an amp hour reading.
Know what you are buying and using. There is a significant difference in longevity and performance between the 2.
True deep cycle batteries can cost 40% more, whether they last 40% longer is up for debate. IMO it is worth it to have Known quality versus something that was built to wally world's price demand.