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Only one “clean out”? If you mean only a single 3” termination fitting, then there will be 2 separate valves. Most likely near the tank outlets, look for a 3” valve for the black and a 1 1/2” for the gray.
Since you bought this at auction, there’s no telling what the previous owner may have done. I would get a mechanics creeper and start tracing drain lines back to the tanks.
Not all trailers are plumbed the same. My previous fiver (Jayco) had 2 tanks and 2 separate drains. Current has 2 tanks and a single drain, with 2 valves.
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2012 Copper Canyon 273FWRET being towed by a 1994 Ford F350 CC,LB,Dually diesel.
Airlift 5000 bags, Prodigy brake control, 5 gauges on the pillar.Used to tow a '97 Jayco 323RKS.
Now an RVIA registered tech. Retired from Law enforcement in 2008 after 25+ yrs.