While I won't be able to tell you the exact length cable you'll need I can help point you in the right direction. The first step is going to be to figure out exactly how your current valve is mounted so that you can remove it and choose the correct replacement.
The LaSalle Bristol Cable-Actuated RV Waste Valve for Black Water Tank # 34466N4AB36GM you were looking at is a Spigot to Hub design. The spigot end with sit inside of a hub and the hub end with sit over a pipe. The hub and spigot... view full answer...
The MORryde X-Factor Crossmember part # MR33WR attaches to the leaf spring hangers which are located in-between and in the front and rear of each axle. I have attached a photo below to clarify. Therefore, you'll need three for full support on a tandem axle trailer. view full answer...