In the southwest and southeast where it does not freeze it is easy to winterize the holding tank. The simplest solution is to stick a TankToolRx or even a garden hose down the toilet and let it run for 3 minutes to rinse out everything. Shut the valve and you’re done. The built-in sewer flusher does not have enough volume to do much. In areas where a hard freeze can be expected do the same thing except if you have a 5th wheel or trailer that you suspect has a belly….which is likely. In this case add a gallon of RV anti-freeze to protect the tank from cracking. In both cases add enough mineral oil to the toilets to cover the valves to protect the seals and keep them soft. When it is time to use the RV again simply flush out the anti-freeze and add 2 ounces of TankTechsRx for the first use of the season. After that, do not empty the tank completely and add only ½ or 2 capfuls after each dump out.