I suggest you use a hose or a big watering can and put some water down one of the vents (with both gray and black tanks empty.) Then, open the dump valves one at a time and see which tank drains out the added water. Note the answer, and you now know which s which.
Good luck, and maybe I'll se you down the road.
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2014 Fleetwood Storm,
2008 Honda CR-V,
and two cats who enjoy being on the road.
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