something is not adding up
you could almost have a dead short and it would not kill the battery in 5 minutes
we you say fully charged battery, are you saying you test the battery just before you left the dock, just before you started trolling or just before you left you're home to go to the trailer...how long was it hooked up to the boat after charging
jave you tried a different charger to make sure it is charged
have you tried only connecting the battery just before you start the boat