Yes even when you are hooked up to a sewer system at a Campground you should use holding tank chemicals in the tanks. Even when you are hooked up to a sewer you leave the dump valves of your RV closed for the reasons listed below.
1. You leave the dump valves closed so that sewer gasses do not enter your RV.
2. You leave the valves closed so that waste matter will not harden on the bottom of your holding tanks which can cause problems with efficiently dumping your tanks and promote the growth of odor causing bacteria.
When the tanks get to about 3/4 full you should dump the tanks (black water first and then grey water tank). If your RV is equipped with a built-in tank flush unit or if you have added an after-market holding tank flush unit to your RV, that would also be the time to flush the tanks.
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