If you thought it was 240vac at the dealer, it was probably a 4 pole RV 50 amp 120vac system. RVs use 50 amp and 30 amp plugs. The 50 amp is TWO 120 volt supply legs that share a common return/neutral. So although it is called 50 amp, it actually totals 100 amps. The separate legs power separate busses (grouped circuits) so if one leg isn't supplied, its associated circuits and appliances are not either, like one air conditioning unit on one leg each when you have dual AC units. Most 50 amp RV adapters link the supply to both legs which means limited power to all circuits. You MAY not be sending any power to the converter to charge the battery.
Bottom line, your adapter may be hosing you.
Have you tried charging the battery disconnected from the RV, measure the charge, let it set a day, then measure the charge again to check the self discharge rate?
Edit: just reread thread. Brand new, take it back to the dealer to repair/replace battery. Brand new, dealer problem to solve!