My black and grey tank is in a similar condition as your's and I am reaching the point in my restoration where I am about to tackle the plumbing. So I am interested in what you end up doing here also. My black tank is shot and will be replaced with something custom, and I am also missing the joint between the black and grey, and the junction between the two appears to be damaged. (Like you said, they probably ran over something.)
Our gas-electric fridge was removed before we bought the camper. But, we decided to just get a mini-fridge and use it on shorepower. Depending on how you use your camper, you might consider just getting a 110v fridge and using that when you have hook-ups available.