I've been charging my trolling motor batteries until the voltage hits 12.8 but am confused if this is considered fully charged.  What is the proper voltage for a fully charged tm battery?  I'm concerned about overcharging them but is 12.8v a full charge?  

my charger shuts down and then maintains the charge on the battery.  When I hook it up to my fish finder, (a cuda 242) it gives me a reading of 13 volts, which to my knowledge is a a full charge on a battery.

I always thought it was 12.4-12.6, but I don't know.

A fully charged wet cell has 2.1volts per cell. If after charging you wait an hour to test I think you will find it is 12.6 volts, while still warm from charging it may read slightly higher.

It depends on the battery and the electrolite used.  Most new/good deep cycle batteries will read 2.14 volts per cell (18.4 volts) when fully charged and allowed to sit 24 hours.  Many maintenance free cranking batteries will only show 12.6 volts.  Before taking a voltage reading of a battery, it should be allowed to sit at least 24 hours after charging or you will read residual voltage left from charging.

It takes a minimum of 13.8 volts to get any charge in and more to actually charge most batteries.   13.2 - 13.6 (depending on type battery) is required just to maintain a charge in a battery while stored.

If dealing with AGM or Gel batteries, never use your standard automatic charger.  These batteries have to be charged with a charger that controls the charging voltage.  You never want to charge those any higher than 14.8 volts and normally in the 14.4 to 14.6 volt range or you will damage the battery.  Your standard battery charger will charge at over 15 volts and some as high as 16 volts.