Hmmm, I think I would also check output of the charger just to be safe. Since those batteries were only 4 months old, and both of them bad, check it's output as well. Another thought would be the alternator is not putting out enough and those batteries go down while traveling, they should not, but, the genset comes off those batteries, and there may be other things which do as well. Additionally, there is a solenoid which kicks in/out so the house batteries are charged after the chassis batteries are charged up, and this may have failed, which means it is closed all the time allowing both banks of batteries to get charging voltage all the time. But that is the way I think it is supposed to work. EM know for sure and can verify that statement of not.
And batteries can be poorly made and they just failed real early. It happens I guess. What brand of batteries were they?
Monty & Janet - 2007 Alpine APEX 40 MDTS
S/N - 75715 - Retired - Master Certified RV Tech