Ok, just tossing this out there, but by any chance could it be rodent pee/droppings or some combination? Sometimes looks like a thick, dark oil after some time. I first noticed it a long time ago under a storage bay door on our rig with about a 2" dark "oily" spot on the concrete underneath. There is NOTHING mechanical anywhere NEAR that area. Upon investigation, there was evidence of mice in that bay/area.
Did you find any other evidence of critters inhabiting your motorhome over the winter?
Those little buggers can get anywhere!
Good Luck,
~Rick
