As long as one uses the RV a couple of times during the season and throws RV Antifreeze into the traps for the winter, all is fine.
Why spend extra on a rubber flap that'll dry out and leak?
The P-trap uses constantly replaced water to form a gas-tight seal. Why mess with what works very well, isn't prone to failure and will last the life of the rest of the RV with nearly no maintenance?
TT 2015 19RD "TheJayco"
TV 2003 F-350 "Montblanc" - Housebroken chore truck
Sitting in The Cheap Seats.
And proud of it!