We have a Forest River Sunseeker. The black water valve was leaking so I purchased a new one. We discovered the drain pipes had been ABS glued together presumably after the valves had been installed so there was no play or clearance to remove the old valve. I hauled the main drain pipe out of alignment and my wife removed the old valve and seals with difficulty because it was a really tight fit. The problem: how to install the new valve if it was so difficult to remove the old one.
My wife solved the problem. Instead of installing the seals on the drain fittings she installed them in the valve and then sandwiched the valve between two stainless steel spatulas. I hauled the main drain out of alignment again and she slid the new valve in place. The spatulas kept the seals from getting pinched or sliding out of position. We positioned the valve so we could put two of the bolts into the housing to make sure we had a good alignment. My wife then slowly slid one spatula out and then the other. I tightened the 4 nuts and bolts and tested the system. No leaks and a perfectly operating valve.
The only change I might suggest is to coat the gaskets with Vaseline before putting the valve between the spatulas. I think this mightease the removal of the spatulas.