you can UPgrade all you want
BUT if you continue to use power and recharge as you have always done
you will kill another set of batteries, it will just take a little while longer, thats all
i have no idea by what you really mean when you say 3-4 days until we need to recharge
assuming you mean, it took that long until things like the fridge quite working and the water pump and lights are to weak and dim
then that is TOO long and the batteries are TOO LOW
if you mean it takes that long to get to 50 percent SOC, then you did very well but might not be fully recharging the batteries as you did not mention how long you run the generator to what charge point was reached aka amps and volts