If you are sure about the battery then the next place to start is the charging system. You indicated that the charging system indicated a discharge issue, and you landed and made a second flight with the same condition.
The engines started fine for the second flight? No battery issue after having the discharge indication?
During discharge situation, could you determine if it was both front and rear or did you try to isolate it besides the regulator select? Did you try switching off the alternators one at a time?
Here are some threads that may reveal some insight. Just be careful that the system is the same as yours as Cessna did some changes throughout the years. Your '68 is different than the newer G models
http://www.337skymaster.com/messages...ight=discharge
http://www.337skymaster.com/messages...ight=discharge