Most of these Thor products now have the drain line as well as the vent line plumbed out the bottom of the coach. The drain should have came with a valve installed from the factory, the vent is/was installed without a valve. The problem has been the vent line, once the tank is filled the vent begins to siphon & continues til the water drops below the line. The factory has added piping to some & created a loop in that vent line, others have just added a valve to it, which is what I've done, but you must remember to open the vent valve when filling so as not to rupture the tank. The vent was designed to be attached to the small screened port next to the fresh water fill opening, but in their infinite wisdom thought it made too much mess when the tank was full & ran out that small port, you could plumb it where it belongs also.
Danny & Linda
Sold our '13 Thor Redwood 36 FB
Currently RV & truckless