I don't know where that goes... looks like a rubber hose going into the trailer, though the rubber 'gasket' screwed to the pan. Take a screwdriver and see if you can scratch a small spot.. to see if it is copper or plastic line. Remove the 'gasket' and look inside... post a picture of your investigation..
Propane lines run under the trailer so that if you get a propane leak on the supply lines, since propane is heavier than air, you have minimal chance of it getting inside the AS.
That line appears to be a LP line because the fitting looks like what you usually see on one... but, AS does make changes...