This sounds like your house batteries have gone bad.
Using a voltmeter is not a good test of a battery, as even a bad battery can show 12V.
The first thing you need to do is to take your batteries in to have them load tested - which most auto parts stores will do for free.
Once you have verified your batteries are good, if the set will still not start, please let me know.
I will need the full model number of the genset.
THE MODEL NO. IS 5.0 BGE-FA/26100P (40.0 AMPS Gasoline) ORAN EMERALD PLUS 5000 GenSet. AS I STATED BEFORE THE BATTERIES HAVE BEEN CHARGED TO "FULL" ACORDING TO MY BATTERY CHARGER.
Yes, but as I said, even a bad battery can read 12V.
The first thing you need to do is to have the batteries load tested to make sure they are good.