Originally Posted by Boxster1971
All drains have a HEPVO valve instead of the conventional goose neck trap. So my guess is as you tried to drain a sink water it filled the pipes and would not drain. The HEPVO valves in shower and bath sink prevented a backup in those areas.
Hepvo Waterless Valve
Wish I'd have known about this before. We don't need to pour RV antifreeze down each drain when winterizing to displace water in a p-trap, just down one
drain to get enough antifreeze into the gray tank to protect the seal on the dump valve.
The forums are a great resource for not-so-newbies, too!