Take the trailer to a dumping station and remove all debris. Dump out both tanks. There is a black and grey tank. The black tank holds waste, and the gray tank holds sink and shower water.
Fill the tanks with water. Return to dump out the tanks again. This will flush the system of any debris that remained from the first dumping.
Pour several cups of water softener into each tank. You can do this by pouring it down the sink and commode. You will need to flush the toilet to get it in the black tank. Softener helps keep debris from sticking to the tanks walls. This will help with future cleanings.
Pour a bleach laundry detergent into the toilet to disinfect the black tank. This will remove any smell in the tank that might make its way into the camper.