We have 3 vents on our TT and I have replaced them with 360 siphon and all had vent pipes. Most TT have vacuum pressed roofs and as such don't need a attic vent, one way you may not have a pipe at the roof is if your vent pipe has fallen in to the tank. You might look down the hole with a flashlight and see if you see the end of the pipe. Gray tanks will also benifit from the new cover. Good luck.
2015 GMC Denali HD ,Duramax with Allison trans. 2500
2014 Rockwood Signature Ultra Light Diamond pkg 8315BSS.