tbh one of the easiest ways might be put the newish battery on a known healthy car and that cars battery on the C5 and run it, see if it 'behaves'.
if the C5's stamina improves, it will point to your charger as the most likely at fault, and with 3 batts tried - it would be where i'd suspect the problem is.
i hope you know, lead acid batteries need topping up once a month, while and Gel batts (low maintenance ) require a specific charger designed for them, or the high uncontrolled charging rates from a lead acid only charger can ruin gel batts real quick.
regs
Alan