Several things can cause the 12VDC to dim momentarily. First, how old is your battery? Have you checked the water level in each cell? Are you hooked up to AC when the 12v dc is acting up? Old batteries will cause the the lights to dim when the charger is kicking in or off, low water can also cause the this happen. A weak cell in the battery will also contribute to the problem. If hooked up to AC, look for something that starts and stops on the AC side of the source to RV. A source of elimination can help find the problem. Start by turning off things such as fridge ( a 12 V draw source) Next try the hot water tank system ( turn off) to see if the problem still occurs. Try other lights to see if the problem still is happening. A bad bulb in one light can also cause the light to momentarily dim and brighten. If hooked to AC source when this happens, the AC source can be below 110 VOLTS AC (low voltage to your RV). Last but not least is the DC power supply may not have enough AC voltage to make it perform correctly.