I had also opened my camper this year to be greeted by a "urinal cake" smell!!! My wife was not happy!!! We thought we forgot something in there also, NOTHING!!
As we "unwinterized" the camper, flushing and sanitizing the tanks and lines seemed to help the odor.... not all of it though
Anyway, I had just saw something on TV that said leaving a half an apple in your car overnight will remove smoke odors, I wonder if it might work with mine and your odor....
Good luck and happy camping!!!!