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Bless RV manufacturers, for some reason they seem to like to do things on the cheap. If it cost a penny more for a dump valve handle that was anything other than black, they'd be sure to justify making both the gray water and black water handles black by saying they couldn't afford the colorful one.
This leaves RVers with a "new to them" RV, or who've just can't remember, in the unenviable position of trying to guess which lever to pull first. If you're new to tank dumping, here's something to learn and remember: Always dump the black water first, close the valve, then dump the grey water. Why? Because the gray water will then flush the nasty Klingons out of the sewer hose and make your life, oh-so-much easier.
OK, here's the case of knowing which valve handle is which. If you have an owner's manual for your RV look there first. Chances are, if you bought a used RV, you don't have the manual. And chances are great, if you bought a used RV, the company probably doesn't exist anymore. Next up, if your rig has "compartmentized" your dump valves (a lot of motorhomes do this), look in around in the compartment and on the inside of the compartment door for a diagram that may explain in Egyptian-style hieroglyphics which handle dumps which tank. Don’t understand hieroglyphics? Take a picture of them, and tell your significant other this is a great reason to take that vacation to Egypt to find a translator.
Seriously, if the above two steps fail, we're down to down and dirty. Yep, crawl down to the ground level and hope against hope, that unlike the poor fellow with the 2008 Pilgrim Lite that your bottom isn't covered and you can see your pipes. Well, be careful about that. Anyway, if you CAN see the pipes coming down to the dump valves, a giveaway for what's what is that the black water line coming to the valve will be a BIG one, that is 3" in diameter, while the line to the gray water is smaller, in the neighborhood of 2" or maybe even less.
And what if you find that you can't see what size pipes you have? Then it's going to be (drumroll please), a CRAPSHOOT! Take your rig to a proper dump station, hook up your sewer hose to the outlet port, and repeat after me: "Eeny-meeny, miney-moe," while pointing your finger back and forth between the two levers. With an assistant carefully observing the outfall of the sewer hose, pull one (just one!) of the levers, and have the assistant report what comes shooting out of the sewer hose. Once you've established what's coming out the hose, you can associate the lever you pulled with the tank it's attached to.
Now, using masking tape and other appropriate material, mask off the areas surrounding the dump lever that's associated with grey water, and spray paint that handle with a color other than black. Gray might be an appropriate choice here. That way when dumping your tanks you can repeat the old acronym, TBOF – "Tug Black One First," and you'll be dumping your tanks in the proper order.