the switch should be a simple on / off switch. either 100% or 0%. no way it should only supply 50%. were you comparing the lights and fans to when you were connected to shore power vs when you are on the battery? if disconnected from shore power I would not expect the lights and fan to even come on with the switch disconnecting the battery. if you then connected the battery and there was a notable lowering of them it sounds like the battery is not fully charged. now, here is a possible explanation. if you are on shore power do you have the switch set so that the batteries are connected. they have to be in order to get recharged. if by chance you are on shore power and the batteries are not connected they will not get recharged. sorry for the nomenclature but I simply cannot remember if the batteries are connected or not when the switch says on or off.
2015 cardinal model 3825fl
2015 dodge ram 3500 dually