This is done quite frequently here in the Yuma, AZ. area on fivers that aren't moving. Removal of black tank can be fairly simple or a real PITA. Remove the RV pottie, drop the black tank, clear the hole that was used for the connection from the pottie. Use ABS black plastic to the sewer connection. Use the already in place water supply line and modify it reach to the new tank. Here in this area with so many RVs there are several plumbers that do just this. A few use a mobile RV repair person. Cost is dependent on labor rate and difficulty of job. If the fiver isn't going to move this is the only way to go.