The roof vent on your trailer is most likely for black water tank. The Lippert Components 360 Siphon RV Holding Tank Vent Cap part # LC389381 has a base with a 4" outer diameter. The holes most likely won't line up with your existing roof vent but you can drill new ones, jut make sure to seal everything up with part # 34427034145-1. The Lippert Components 360 Siphon RV Holding Tank Vent Cap is designed for a 1-1/2" or 2" diameter vent stack which is a standard size so as long as this is... view full answer...