The vent pipe has to be sealed to the tank otherwise it will leak sewer gasses under your rig. If you trail the rig with other then an empty tank, it could leak everything you flushed. It either has a rubber seal at the penetration or is glued with abs plastic cement. I have seen both ways. Find out which wall the vent stack is in and open it up to expose the stack. Check how the vent is attached to the roof. You will have to carefully remove any caulking and hardware. If it goes straight up to the roof and uses the rubber seal, you may be able to pull it upwards and disengage it. Check how much is in the tank, it only needs to be an inch or two. Cut off any excess. Lubricate the end with Dawn dishwashing soap, then carefully reinsert it into the seal. Eventually, the soap will dry and give a good seal. Reattach any roof hardware and recaulk with self leveling lap sealent caulk! It is available at your local RV store. DO NOT use silicon caulking.
If it is glued to the tank, you will have to cut out and figure out how to glue the pipe to the tank.
If the vent stack is not a straight shot from the tank to the roof, you will have to cut it between the tank and first elbow, shorten the inserted end, then reattach it to the tank and, using an abs coupling, to the other end of the stack. Big box home improvement stores carry abs fittings and glue. Be sure you get ABS cement and fittings. PVC cement and fittings will not work.
2013 Rockwood Signature UltraLite 8319SS