Well if it is reading 12.8 volts the battery is fully charged and therefore not the problem. Make sure that the battery is not connected backwards in the RV, positive from the battery should be going to the coach main converter fuse panel. The negative lead from the battery is connected to the frame of the unit and you can normally see where it is connected to the frame close to the battery. Also make sure that if you have a battery disconnect that it is not operated cutting off the battery from the coach.
Follow the positive lead from the battery and you should find the fuse you are looking for and the reset button. On a travel trailer many times the fuse is on the underside of the frame going to the coach.