Wow! Pictures might be in order. Typically, the black and gray tanks in coaches such as yours and ours have extremely short 3" and 2" respectively black PVC lines to the two dumping valves, and all of that is typically in the wet bay on the street side of the coach. Do you have a curb-side toilet? Or are you talking sinks or washer/dryer? Regardless, normally all inflows to the tanks are PVC piped into the top of the tanks, and the only outlfow is the dump PVC piping at the wet bay.
Is yours not plumbed like that??? Or are you talking about the storage tube for the sewer hose? If that's the case, there are many good options available at any store or dealership that sells RV supplies.
The valves can fairly easily be replaced. If I were doing the work, which I did once, I would be sure both tanks were totally emptied, and I would do a fair amount of flushing with fresh water to minimize the "uck factor". I think you will find that four machine screws will allow each of the valves to be slid out of their positions without having to do much with any of the rest of the plumbing.
Jim & Angie, Boise, ID
2021 Entegra Esteem 27U
2017 GMC Acadia Denali