don't know if i am right maybe someone else on here will chime in i'm sure but ,
the deep cycle batterys do have a limit on the number of times they can be recharged not that they just wont go but the run time will keep getting shorter and shorter.
and it's hard to check voltage on a deep cycle for the reason that they hold 12 volts til dead unlike say a car starting battery that drops voltage as it goes dead .
and maybe like you said it very well could be your charger . hope someone with more knowledge jumps in here and either supports me or tells me i'm wrong and tells you right lol.
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hope i helped just my experiance ,
Scott