If it's like most fresh water tanks, one of those lines is a vent, the other an overfill. On most the vent is supposed to be attached to the small screened port next to the fill port, but Thor engineers in their infinite wisdom choose to run it out below so as not to make a mess in the convenience center. Their fix for siphoning was to add pipe to that line & loop it up higher, it works. Mine both go out the bottom so I just added pex valves to both lines & have remember to open both while filling, when it starts to siphon close that valve,
continue filling til it comes out the vent, once I've used a bit out of the tank I open the vent.
Danny & Linda
Sold our '13 Thor Redwood 36 FB
Currently RV & truckless