I didn't drain the black tank in a timely manner this past weekend on my 303tg
and sure enough the outside kitchen sink was the first drain to back up...well, it didn't "back up" due to the trap, but it didn't drain until I pulled the black handle. Thankfully that's the lowest point for any type of backup to occur, so it's a bit of an early warning system.
I actually do like that it drains to the black water tank....doing dishes outside, it does allow for some extra "soapy" water to mix in there with the solids, as others have pointed out.
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Dave
Trailer - 2013 Laredo 303tg
Towed by - 2012 Chevy Silverado 1500 Crew 5.3L
Former Trailer - 2007 Rockwood Roo 23SS