Originally Posted by GDMatthews
Thanks for confirming these details. I poured antifreeze down all drains and then confirmed it was coming out of the grey and black systems, draining pink. I didn't realize there was a p-trap involved but assumed complications with a system I couldn't examine with my eyes.
BTW, I'm very interested in your boondocking set up: "470 watts of solar on the roof | 2x6V GC batteries | 1500 watt PSW inverter | Micro Air on A/C | so far strictly boondocking." Can I ask away here or should I start a new thread? Still learning...
Each sink or shower drain has a P-trap, just like in a house. Some use a different type of device, but my camper uses p-traps. The reason for pouring antifreeze down the sink drains is to displace the water out of the p-trap and replace it with antifreeze so you don't experience frozen water in the p-trap which could burst it. From there, the pipes lead to the gray tank.
Also, pour antifreeze into your toilet bowl so that the flapper is covered with a couple inches depth of antifreeze. It is slow to evaporate and will help keep the flapper seal lubricated all winter so that it works properly when put back into service. It also helps keep the black tank odors in their place instead of making their way past a dried up seal and into the camper living space.
If you've got your black and gray tank dump valves open and are observing anti freeze coming out, then you probably don't have to worry about the situation I described in my original post about residual black/gray water collecting there. But once you start using those tanks, I doubt you'll want to be opening those dump valves without a sewer hose attached and draining to a proper RV dump or sewer hookup. At least for the black tank anyway. So in that instance, I like to be liberal with how much antifreeze I put down the sink drains so that the excess gets into the gray tank and collects at the lowest point. Same with toilet and black tank. Dump valves are closed at this point so I imagine any antifreeze that went down there is collecting upstream of the dump valves.
I don't mind you asking solar or boondocking questions, but a new post under a relevant title will help keep the discussions on topic and helpful for others browsing the forum. :-)