So your shower or sink is draining much slower than normal and you want a quick fix. First of all, DO NOT run a harsh chemical like Draino in a camper with holding tanks. The chemical will not likely damage your pipes, but it could damage the rubber seals along the road from your sink, past the valves in the holding tank and even to the hose you use to drain.
Give this treatment a try: 3-4 Tablespoons of Baking Soda followed by 1 1/2 - 2 cups white vinegar. Let that sit for about two hours. Next, pour a large pan of boiling water down the drain. If it still runs slow, do it all over again.
Hopefully that fixes your problem. If not, then it might be time to call in the professional Camper Service Technician.