That is the return line from the fuel injectors. The fuel system sends the fuel to the injectors and they pop off fuel to the cylinders at a certain pressure. The excess fuel is returned to the tank through this line. It should be an open line to tank. Trucks and mine equipment has the engine above the level of the tank and do not have to worry about this back flow. The motor home engine is below the tank level so I guess you are getting back flow from the tank through a leaking fitting. I am not sure what type of fitting you have. We used hoses with what is called a banjo fitting with some copper washers used for seals. It sounds like you have plastic lines that are sealed with a little brass ring around the hose in the fitting. If it is that kind you can just cut off the line and slip a new ring on it and put it back together if that is where the leak it. Those rings are not too reusable because when you tighten it up it forms to the fitting. If you take it off it is hard to get it exactly back into the same place and might leak. That type of fitting is found at all hardware stores. We used these fitting mostly for air or water.
Ron KB9TF, Fulltime
2008 Tiffin Phaeton 40 Foot QSH
2011 Honda CRV with Blue Ox towbar and brake