To the OP, don't know how old those two batteries are
But sounds like "loss of capacity", they have been used hard each trip and not fully recharged EVERYDAY, it's called progressive sulfation
Even if you fully charged them with an external battery charger, they would not be the same as new
Batteries should be recharged as full as possible each and everyday
That means a good run of the generator with a good converter or external charger
In your case this could be four to six hours,
I recommend four new batteries, and some solar, to help reduce generator time
Also a battery monitor system "amp hours", you want to monitor what goes out and what gets put back in,
The idiot light cluster on the RV monitor panel is useless, for knowing if your batteries are really charged or how much power you really have stored in there to use
