Battery question

So my battery was almost completely discharged. We hooked it up to a separate battery charger and after about 20 minutes it was slowly making progress to recharge the battery. We disconnected the separate battery charger and plugged the trailer in through shore power to continue charging the battery. It was plugged in to shore power for a little over 24 hours. When we disconnected shore power after 24 hours, the trailer had no power. The monitor panel inside had no power to show the level of charge, even the carbon monoxide monitor had no power. It's starting to sound to me like something other than the battery but I'm definitely no expert. I know the dealer is going to want us to take it there but I don't like that option. We did not have a good experience when purchasing the vehicle from them and its not near our house. Are there other options? We have closer RV dealers but do they all service and do warranty work for every manufacturer? We will not pay anything to have this resolved. This is a brand new unit. Sorry for the frustration. I appreciate all of the advice.