Need help with battery issue.

Joined
Jul 5, 2013
Location
Tuscaloosa, AL, USA
#1
The other day on the lake I noticed my trolling motor wasn't pushing the boat around like it normally does. Previously, I had left the on-board charger plugged in for over two weeks, so I knew I didn't forget to plugin after the last outing. When I got home I checked the voltage and it was around 10.5v. I plugged in the charger and after one week I am only at 11.65v. It should be higher than this.

I have two group 27 batteries wired in parallel and they both appear to have been purchased at the same time some years ago. How likely is it both of them would go bad at the same time? Has this happened to anyone before? What else should I be checking? I don't want to dump $300+ on new batteries if this isn't the problem. Obviously, I will have the store check them before purchasing but I would like some advice before pulling them out.