A short tailpiece of pipe that is threaded on only one end of the pipe is threaded into the tank bottom. Thread dope is applied to the threads before it is threaded tight into the tank bottom. You next dry fit you slip joint plumbing together and mark the joints. (Hard to see with Black ABS pipe.) When things are lined up to where you want them to be you then disassemble your plumbing clean the joints and glue it all back together.
With most glued drain plumbing, if you have a problem if can cut a piece of pipe out and join it back together with a slip joint coupling. With the Scamp
drain the couplings are so tight together you don't have room to add a coupling.
If you look at the picture in post 17 you will see that when you try to unthread the pipe from the bottom of the tank the 90 will hit the frame rail. This is how it is done in a front bath since it is so tight a fit. In a side bath you may have room to unthread the tailpipe by removing the gate valve but I think you still may have the same clearance unthreading issue.
I think you and Carol may have side bath Scamps that's why we are not quite on the same line of thought. I have been dealing with Scamp
FB plumbing most recently and on those trailers the pipe has to be cut to unthread the tank tailpiece.