bad bat. gauge?
 

So along with all the other issues ive been chasing down i think i now have a bad battery gauge.

When i bought the boat last summer the gauge always was lower then it should be, but i figured the alt's were not charging at full power or whatever. fast forward to this year i had an alt. go bad so i swapped them both and went with a 1 wire setup. now the gauge reads at 8. The alt's are charging i checked across the batteries and im getting 13.5 at idle.

before i tear apart the dash (the last thing i want to do right now) does anyone have any thoughts other then yeah its a bad gauge?
I dont know if there is a dedicated signal wire or not (again havent taken the dash apart) that could be the problem.