Sewage in Camper

First off my sympathies to you for this disaster. I would not accept anything but a total loss on the unit. Start shopping for a replacement and don't accept a lesser settlement where you are left to come up with funds to replace it. Secondly, thank you for posting. I never really gave it much thought about leaving the grey water open when at full hookups. I will now. I do now shut my pump or city water off when we leave due to having read too many horror stories but flooded campers upon return. Good luck and let us know with a followup as to what the outcome is. Meanwhile, the campground should be renting a trailer for you to enjoy the summer.